The specific dimensions of the upper and lower have almost everything to do with this.
From a reliable maker, uppers and lowers should be snug at first. It’s almost certainly not something to do with the pins, unless you go with fancy - and not necessarily standard sized - pins. But as the receivers get broken in, everything gets much easier to connect and separate.
I once bought a Rock River complete upper and a RR complete lower to go with it. When new it took some serious “tapping” to get the upper and lower to close, and more tapping to get the takedown pin all the way in.
After a number of cycles, my build settled down into a perfect fit. Rock River (at least used to) mills their uppers and lowers for a near-interference fit at the lugs - on purpose. That allows the parts to wear into each other, and after that, it’s like butter to put them together or take them apart.
It’s OK to need a bit of tool help to get the takedown pin started out on a brand new rifle, or a new build. Give it time and it should be much easier to manipulate.