here is my understanding. When you buy a firearm, the item that is numbered primarily is the firearm and all other pieces are parts. the single part of a firearm that holds trigger and firing pin usually is the gun. no other parts, unless subject to individual regulation by their nature, are regulated, if that makes sense.
In the case of the ar, iirc, the firearm is the upper receiver assembly, not the lower with trigger group and fire control.
I DON'T KNOW WHY.
This really isn't complicated. You should be able to buy a receiver and create a firearm from parts and that legally purchased receiver, because legally, the numbered receiver is already a firearm. You are adding unregulated parts to it.
Human nature being what it is, and law being what it is, whenever you do something that is on the dark side of completely innocent and legal, you are essentially kicking a dragon in the gonads, and you may wind up like willie, crucified, without having actually violated federal laws. You may not violate ANY laws, and you still may wind up being targeted.
Visit the atf. If you still have any doubts, maybe you should have a licensed gunsmith do some of the work, or get a permit yourself.
YOU DON'T WANT TO WIND UP FACING WILLIE'S PROBLEMS. IT COSTS A LOT, CAN COST YOU YOUR REPUTATION, AND MAY COST YOU YOUR FREEDOM. IF IT FEELS EVEN REMOTELY SPOOKY, DON'T DO IT!