The problem is, no one would pay you anywhere close to $20 per mag body.  You'd be very lucky to get any decent volume at $5 per mag body.  Which is why no one has done it yet.

When AR mags cost $100 each (and someday they will), then someone will start making replacement mag bodies.

-Troy

What stops them is the threat of losing their lucrative government contracts.

Right now, 95% or more of these companies' income come from government contracts.  There are all sorts of ways they could lose these contracts; either from direct political pressure, or from some nutjob who was found to use "evil, banned magazines made from parts made legal from the 'parts loophole.'"

Then, factor in the fact that these companies are geared up to make complete mags.  Changing a manufacturing line costs big money and causes big hassles, even for something that sounds as simple as "take x number of mag bodies off the production line before assembly."

Finally, the mag companies are only set up to sell to large companies who will order 20-50,000 mags at a time.  They have no retail sales infrastructure, nor do they want to pay for one.

Basically, the current companies would probably not make a profit by selling mag bodies separately, and could risk their entire business.

Again, once mags cost a LOT more than they do today, then someone WILL find a way to make mad bodies available, assuming it is still legal by then.

-Troy