Because buying a 3000 3D printer is much easier than buying a 40 dollar gun from a shop. Right?
Where do you live? Africa?

The only semi-reliable gun you can get at that price is a second-hand Chinese AKM copy (and that's because being an AK, it's reliability is a given). And even then, they only go for that price in 3rd world countries. I can't really speak for the US, but I don't think you can get a decent weapon for less than 500$. And the AR-15 is a a big step up from simply a "decent weapon".
Maybe next you can address the dangers of civil-war vintage restored weapons? Or perhaps the general ease by which a flintlock rifle could be stolen from a museum and the need for additional guards and security meaures?
If you steal a real Civil War-era rifle from a museum, the correct course of action is to sell it and use the money to buy a modern gun.

A Civil War-era cartridge rifle is still as deadly as a modern gun (unless we're talking blackpowder muzzleloaders which were still in use at the time), but I think that the same regulations apply to them as to the more modern guns. The difference is that a 3D printed weapon could be manufactured with just a 3D printer and a bunch of material. That said, I think that good ammunition control can handle those, as stated above.
I mean yeah, you can make an improvised gun on your own and it might get a lot easier with 3D printing, but it's not that hard to make a homemade bomb either. You can do it by accident, even, hence all those explosions at fertiliser factories. But nobody uses this as an argument against 'bomb control', because it's still much harder to kill people with homemade bombs than if you could buy them from a shop and it's pretty easy for the police to pick up on people manufacturing illegal goods.
You can actually buy perfectly good explosives from a shop. You can then proceed to blow yourself and others up with them, a much beloved activity on the 4th of July and New Year's Eve (for best effect, pair with a strong alcoholic drink).

They even make them in a wide variety of casings, yields and flame colors.

Actually, making a bomb from fertilizer is downright trivial and much easier than buying a large quantity of fireworks in the store (not to mention it attracts less attention). Such a bomb is pretty well behaved if you do it right, which isn't that hard, either. It doesn't mean it won't blow you up if you do something stupid or screw the production up, but it's a secondary explosive (meaning that usually, something else needs to explode first for it to go boom), not a primary.