+1
Unlike a bolt-action rifle for instance?
On the AR-15 design, the barrel, bolt, and sights are all firmly attached to the upper receiver.
The fire control parts are contained in the lower receiver.
It doesn't really matter much what the lower receiver does.
Because it has very little if any influence what the upper receiver, barrel, bolt, and sights are going to do.
Lots of shooters like tight upper/lower fit, but it's more a matter of things rattling around then any increase in accuracy. That can be fixed with a $1.99 plastic accu-wedge.
The other thing is, If you somehow pinned the barrel to the lower receiver, there would be no way to open the gun up for cleaning & maintenance easily in 5 seconds.
rc