The next question is why.
Full and semi auto weapons are over 100 years old. The AR-15, in particular, is 70 years old. However, we don't see school shootings really pop up until Columbine in 1999, and that happened while the federal assault weapons ban was still in effect.
Of their weapons, only the Hi-point carbine would be covered by the AWB. The company has manufactured a version that skirts around some state level AWBs. The rest of their weapons were pistols and shotguns, would be difficult to ban outright, plus improvised bombs made of commonly available stuff.
We have cultural problem with school shootings, and it didn't start until the late 90s. If it were just availability of certain tech, it would have started decades earlier.
A huge issue is the right wing itself. Several shooters cite conspiratorial nonsense spread by the likes of Alex Jones. At least one of the Columbine shooters was praiseworthy of Nazis in his journal, though the exact motives have been elusive. Regardless, the takeover of John Birch Society thinking on the right plays a role.
It's probably that they would be paying it without the tax benefits they do for actual employees. So they never bother. In their mind, not getting a tax benefit is the same as banning it. They believe this because they are stupid.