And the part that is considered "the gun", the lower receiver remains the same, no background checks or anything required to go from a 5.56 to a 9MM or something else with more availability. And then you have the whole AR-10 rabbit hole... These are some of the technical reasons it isn't as easy as you and Tyrone think.
Look, if it was as easy as you seem to think and without a host of easily foreseeable and unforeseeable consequences, I'd be all for it. I don't think the general public should have access to these weapons and especially not deranged 18 year olds. Unfortunately, it isn't that simple.