I never understood why the ATF defined a "gun" in terms of its lower receiver. I assumed that it was because such a thing was difficult to make outside of a big gun factory, which would provide a decent point of control for ensuring that firearms would be sold only to people for legal purposes. (Yeah, that didn't work either, but that's a different question.)
Because it's not a wear part and it's easy to serial number. It's the restricted part. People need to re barrel guns all the time, barrels get shot out, people need to replace their bolts, people want to replace the stocks to customize it, the part that stays the same through all of this tends to be that receiver (or frame in pistols). The idea is to give you the freedom to maintain, repair and customize your gun. In the case of ar-15s its also the part that determines whether its a machine gun or not, go around with a lower receiver with the third fire control group hole for the auto seer drilled out and you have a machine gun whether you have a semi or full auto fire control group in the gun.
But guns don't kill people: fast-moving bullets kill people. You're not going to regulate chunks of lead, but it seems not unreasonable to regulate the bits that explode, e.g. gunpowder and the bullets that contain it. I find it rather odd that I can walk into any gun store and buy explosives, in bulk, with few if any questions asked.
Ok well you just said something about not being able to regulate chunks of lead then follow up by suggesting they regulate bullets... those same chunks of lead. as for the gunpowder, do you have any idea how easy it is to make black powder or even gun cotton? The difference between modern smokeless powder and something like gun cotton is all about how controlled the burn is, if everyone was making their own gun cotton and burning that instead of modern smokeless powder you'd see a lot more explosions instead resulting in injured operators instead of the nice controlled burns... interestingly enough in normal use they undergo deflagration instead of detonation (ie they aren't acting as explosives rather they simply burn)