That and to be honest - they would not sell like crazy. There are a few exceptions..but most would find a very small (albeit very dedicated) audience.
There wouldn't be a decent community to support games featuring a full role-playing theme/genre. Its a neat thought, and hell yeah I'd get on an M41-A Pulse Rifle, to kill some aliens....but we have a hard enough time getting people to wear distinctively different camo, let alone sci-fi get-ups.