I could see why "people are making guns" would be at the top of the list of politicians' worries in places where there are almost no guns, and people want to keep it that way. But in the US?

Indeed. Don't want people making guns? Ban the making of guns. Banning the production of guns using a 3D printer makes zero sense, should ban CNC machines too then.

The gun industry does not earn anything from 3D-printed guns, so those kinds of guns are "free game" for the law makers.

Exactly. In the US you don't even need a license if you want to manufacture a gun for yourself. The idea of it being made illegal is far from reality.

Even then it makes little sense... 3d printers are just tools. They can be used to print dangerous items, or parts of dangerous items in the same way a saw or hammer could be used to make something dangerous. To some degree this is just a problem with human nature – some people are going to want to harm people and will create or acquire items which do that.

Perhaps if it was literally as simple as downloading a model and pressing print, then in 20 minutes you had a fully working automatic rifle this would be an issue, but that technology simply doesn't exist today.

In reality if your goal is to acquire a weapon which can do lethal harm to someone you just wouldn't print a gun. Even if you wanted to kill multiple people in a place like the UK where guns are illegal you still wouldn't print a gun because you'd probably be better off just getting knife than printing a crappy gun and trying to source an effective propellant, etc.