I get it if you live in mansion... but I have *NEVER* had an issue with my wifi signal suffering anywhere in my home. My router is not even centrally located in my house either... and in the farthest point in my house... I still have enough of a good signal that I can play streaming VR from my PC in the living room... in the room way in the back, on my Oculus Quest without having an unplayable experience. It perhaps has a bit more lag... possibly due to a few more packets getting dropped and needing to be retransmitted causing a bit more latency, but not terrible. So maybe for *THAT* specific solitary use case it *MIGHT* make sense to have that extra AP... but Meh,... I'll just keep playing Oculus Quest Wireless Streaming VR in my living room near the router.

My house admittedly is small... but not *THAT* small. My house is 1100 Sq feet.... but like I said my router is practically speaking corner of my house. Even at the furthest point, I still get a decent signal. I can't imagine how big someone's house would have to be for them to not have a good signal anywhere. Granted, my house is a one story home...and is made of wood as opposed to concrete.

IS this just a "Go be poor somewhere else", when people insist on getting multiple APs? *OR* is it a load balancing thing they're doing?
A lot of the people on Youtube that I see demonstrating installing multiple access points in their home... dont exactly have gigantic homes either.

I feel like your home has to be in excess if 3000 sq feet before it becomes an issue.
I have relatives with larger homes than I do, and I don't think any of them have multiple access points... and none of them have ever expressed having issues with their single wireless router with no discrete access points.

My house is 4,100 SF.

I have a router, and 2 additional access points.

I tried to get away with one, on the main floor. I couldn't get wifi in my gun room downstairs or my daughters room upstairs.  The owner's suite also got pretty crap reception and would get patchy streaming at times.  Note this is with an AX6000 (aka Wifi 6) router, albeit Wifi 5 clients, so I was getting decent load balancing, but range obviously isn't as good as pure AX E2E.

I have engineered I joists between floors, and all interior walls, and floors are all insulated for sound deadening.  I reckon when I was on the second floor the signal was going through no fewer than 4 pieces of drywall, a subfloor, and at least 7 or 8 I joists, plus 15' of insulation.

I now have three on a mesh network, and I can go anywhere in my house and have blazingly fast wifi, and can fully realize the gigabit ISP service despite having over 40 connected devices.

I tried to get away with fewer. Didn't work.

Others have other limitations, like concrete walls or things of that nature.

Some others just live in uber congested environments where there are multiple SSIDs within range on every channel.  This is also something I have to deal with as I live in a subdivision and on 20mhz channel widths (some people have theirs setup as 40 or even 80mhz) the channels don't allow for much, if any spacing. So that could be another contributing factor.

Square footage is just one part of the equation.  The number of devices being used simultaneously is just as important, as too many devices on one device can become a bottleneck. Larger homes and families tend to have more internet-connected devices.

Depends on the construction of the house too. My bedroom is an addition to the original house and has a brick wall between me and the router. I need to add an access point to get anything to work on that part of the house. Only 2400sq.

My house is 3 stories and built in 1850... plaster and lath walls, the router is in the basement. I had zero signal on the third floor and crappy signal on the second floor.

I disabled the built in FIOS WIFI and have have multiple meshed AP’s on every floor,  and one that covers my yard (detached garage)

I used cheap POE Cisco WAP131’s from evilbay because I hate dead/weak wifi.

ETA: no way would I game over wifi... even a PC 2 feet 2 feet away from an AP gets crappy throughput. Try a speed test comparison on a PC wired and wireless... you may be surprised.

Because some houses have metal walls, microwaves, 3 stories, remote offices, etc.   There are programs that will tell you what your WiFi coverage and dead spots.

LOL.  Six access points at my house, though one is outside in the back yard.  It's all about the 5Ghz Coverage.

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Primary causes - home construction and antenna orientation. Secondary causes - nearby signal interference and saturation from neighbors cranking the gain to max in a feeble attempt to make their single AP work throughout their house. I hope the OP isn't contributing to the latter.

Another reason - I don't want to grow extra limbs and chronic headaches by having max gain APs throwing radiation everything. Having more APs can let you cover weak spots more effectively with lower signal gain. Seamless device handoff works wonders if you have an effective channel plan mapped out for your house.

I add to this (as someone who does it for a living).

In many cases, as single AP is adequate for surfing and even general streaming. You only need 7 Mbps to support a full 1080p stream...and about 28 Mbps to provide a full 4K stream and depending on the encoding method being used, it could be even less. 2.4 GHz can adequately provide that. The issue is when you run into power users who want to see 100+ Mbps wireless speed in all areas of the building. This is something that can't be practically (or sustainably) achieved in a larger environment so many often switch to 5 GHz. The issue with 5 GHz is that it suffers more path loss (about 1/2 the coverage compared to 2.4 GHz).

As a practical real world example, my house is a three bedroom, two bath single story job built in the mid 1970's and comes in right around 2100 square feet. My cable modem is currently located on an exterior wall and my router/AP combo unit are there as well. Covers most of the house just fine...but it has issues in the kids bedrooms (on the other side of the house). 5 GHz doesn't reach and 2.4 GHz is very spotty. Now I could centrally locate an AP in the kitchen or foyer but if I'm going to go through that kind of hassle (running cable) I might as well just add a secondary AP and turn the power down on the two APs. Then I'll have total 5 GHz coverage at maximum throughput which is handy considering my wife runs her photography side-gig off her laptop (and doesn't want to be tied down by wires) and is constantly transferring images to and from network storage.

Now as many may be wondering, why does a network engineer who specializes in all things wireless only have a single, non-optimally placed AP in his house? The only existing structured cabling in the house is the CAT3 job someone from AT&T put in around 1975 and the wife and I originally only planned on keeping the house for a year or two. Now that I've been WFH for the last year and our plans have changed...it has just recently become relevant to upgrade to a wired infrastructure as we intend to keep the house for the next three to five years and re-evaluate.

I feel like I could have written this. Why do I have Aruba gear scattered all around my garage, but my family has to get by with a single mediocre ASUS AP?

This is pretty common with professionals and seems fall under the old saying "the cobbler's children have no shoes" or "the barber's son is the last one to get a haircut". I keep telling myself that I will run some cable through the attic and throw up some refurb APs, but who has time for that?

This.

I also see environments with a lot of CCI and ACI and the prosumer's first response is to add more APs.

There is no "one size fits all" for wifi/radio deployments, and radio itself is magic to most of the population.  

Sometimes a single AP is fine for coverage (Centrally located in the coverage area, quiet channel(s), no single high-throughput device, construction that's conducive to passing RF).  
Sometimes you need a bunch of APs (Lots of other users on the channel/high noise floor, buildings that block RF (tin siding, lath/plaster, wire mesh), or there's a device that's sucking all the airtime (""smart"" tv) from the other users on the AP).

Repeaters, mesh, powerline, etc are all hacks aimed at consumers.  Professionals don't use that stuff.  Pull a cable, it's not hard to do.

If one's working for you?  Don't bother making it more complicated.

My old condo?  with everyone blasting wifi all the time, and the jobless losers streaming tv all day?  yeah, I needed 2 to cover that 740sqft+garage area.

Wiring the heavy-use devices will improve wifi as well.  Get your TV's, media servers, and network-heavy workstations on a wired connection and free up the airtime for devices that need to be portable.

125 year old house.  2 inch x 4 inch x 12 foot studs, plasterboard in most of the house, some plater and lathe in other parts.  In the kitchen, where the main router is located: 3/4" x 5.5" tongue and groove  on the walls and ceiling, over 5/16" t&g, over 5/8" drywall.  3 large brick chimneys.  Upstairs is modern 2x12" floor joists spaced in between the original 2 inch x 6 inch ceiling joists.  The stairs stringers are framed out with modern 2"x12" studs, finished out with 3/4" wood plank for bookshelves in between the vertical supports.

Et cetera.

Just thinking back on it all makes my back hurt.

Multiple access points make things tenable.

My house is the same size as yours OP but is a ranch style and I have 3 AP's.

They are connected to different switches so that when I need to update a switch or there is an issue with a switch I still have WiFi