New hi-cap mags can only be manufactured for military and law-enforcement use. Add to that the fact that foreign-made ones are dirt cheap and you see why no American firm makes mags.
Only magazines manufactured prior to September 1994 can be civilian-owned.
Not to change the subject drastically, but changing the follower and floorplate on a mag is really a cinch.
Not all followers work, though, so you have to be careful to get ones that work in your mag. Some I purchased from K-VAR did not hold the last round in the magazine (the round resting directly on the follower).
In fact, the only US-made followers I could get to work were the orange plastic ones made by FSE (First Son Enterprises).
I'm sure there are other US-made followers that work; I just never found them.