I can't speak for anyone else, but we son't sell a body for 3 reasons:
You're right about the copying thing. We put a lot of extra work in our body, including the inner panels. Hard to do the first time. Easy to copy.
We don't want the body put on an inferior chassis and sold as an ERA. It's the total package, not the body. And our reputation.
We don't want people to be ripped off by someone claiming to sell an ERA body, but actually having a bad copy.