Is there any reason not to use olive oil instead of butter when making roux for white sauce / Alfredo sauce?

Don't know.  You might IM leelaw and ask him.  He's more of a late night crew.  Patty

Olive oil at extremely high temperatures becomes toxic iirc, I use it if I'm cooking low heat applications

Yes, something about the butter when it heats up and the browning effect. I don't remember exactly.

Someone else?

Butter fat breaks down at a lower temp than olive oil.  Olive oil has a low breakdown temp too, but it is too high for making a roux.  It can be done, but you have to be very careful monitoring the temp and stirring constantly (even moreso than with butter).

The roux will usually burn from the higher temp needed to brown the flour and Olive Oil, that is why butter is preferred.  Hope that helps.

ETA:  A roux based white sauce is a Bechemel Sauce.  Butter, flour, salt pepper.

Alfredo is butter, heavy cream, parmesaen cheese, and white pepper.  IM me for a secret ingredient and other recipes if you need them.

I would use butter in the roux. The milk fat in butter helps give the nutty flavor as it browns, you will not get with OV.
Use Parmigiano-Reggiano and a few grates of fresh nutmeg for an outstanding Alfredo

Quoted: Is there any reason not to use olive oil instead of butter when making roux for white sauce / Alfredo sauce?

flavor profile. A roux is, by definition, equal parts fat and flour. Any fat will work but will change the flavor profile.

What exactly ary you making? A straight white sauce(bechamel) calls for a light or blond roux and milk. If Making an Alfredo its whole butter, good parmesan(reggiano) only traditionally. Most folks are more familiar with a cream based alfredo here, whole butter, heavy-or better manufacturing- cream reduced to proper consistency, parmesan. No need for a roux here unless you are making gallons and reduction will not be cost effective.

hth