The rest of the world doesn't really have the same concern or context, because the rest of the world doesn't have the same issues or political/media environment.
Also read in many places buying illegally is easy. How true is that?
If you buy a gun from someone on a classifieds site or friend/family, in most states you don't have to get a background check to transfer ownership of the firearm. This is typically the "loophole" that people refer to when they want Universal Background Checks.
I'm not sure what you mean by buying illegally - whether you mean to someone prohibited from owning firearms (like a felon or something), or buying "illegal" guns... Either way there are usually stiff penalties for owning restricted devices. For example if I were to 3D print an auto-sear for an AR-15 (making it full-auto), that's a ticket to a 10 year sentence in federal prison. Assuming I don't have the permission slip from the ATF. As for prohibited persons buying a firearm, I'm not sure what the penalties are, but at the least it'd be a violation of their release?
Or unless, of course, you're a criminal who is forbidden from legally buying a gun.
Or unless you have a felony, which would make up a much larger share of illegall firearm purchases than teenagers.
"not technically an assault rifle" doesn't matter in the slightest, no one's impressed by this constant pedantry... yes, yes, it's an ArmaLite 15 and isn't technically an assault rifle because it's only semi-automatic. This changes absolutely nothing, mass shooters are essentially cosplaying during mass shootings with this thing because it looks like an assault rifle and it's easily accessible.
the proliferation of modern small arms is one of the worst things to happen in human history