Because unlike modern regressives, our Founding Fathers & ancestors had common sense back when it was still common.
They naturally assumed common sense would remain that.
Obviously, they didn't foresee the rise of lefty collectivist ideology indoctrinating the sheeple & the suffocation of common sense.
And making the case that denying children guns means you can pass "common sense" regulations on the rest of us is a strawman argument.
Besides, I gave my kids guns well before they were 18 & showed them how to use them properly.
Guess what? They are just fine & their guns didn't kill anyone!
Shocking!!
I've got no problem if others do the same as long as the kid ain't a psycho.