They are regulated, but there’s a lot of breakdowns in the system. People passing background checks who shouldn’t, prior offenders passing background checks because local cops didn’t report them to the feds, etc. The DC Navy Yard shooter years back literally had fired a weapon into his neighbor’s apartment before and still passed a background check to buy the weapons he committed the shooting with. I also think if you’re a parent and you leave your weapon accessible by your children, and they go shoot up their school, you should be held at least partially liable. As somebody who is former military, the civilian population gets away with a hell of a lot with regards to firearms. No federally mandated training standards, concealed carry licenses are haphazard and go state by state, and not all states recognize other states’ permits, no federally mandated storage requirements, etc. When I was in the military, if I wanted to go target practice on base with my personal weapons I had to register them with the provost marshal on base, keep the weapons and ammo separate in locked boxes out of my reach while driving to the range, etc. And if one weapon went missing the entire base was locked down; gates closed and nobody in or out until it was located. Civilians get by with way too much.
I think a lot of our problem is loose or missing standards at the federal level, which leaves each individual state to kind of make things up as they go along and not communicate properly with feds when things go wrong.