1. The full game costs money because I need to eat in order to stay alive and keep making games.
2. I don't think there's much reason to ever update the demo. The bugs in it are pretty minor and it's so far behind the latest full version that I'd basically have to redo the demo aspects of it again, which would be a timesink without much benefit. The demo at its current state still gives a pretty good baseline idea of whether you'll like the full game or not, so it does its job.
3. I only released the early access version on Steam because keeping multiple storefronts up to date is a chore, so I only wanted to keep updating one while the game is still in early access, and chose Steam since it's a far more popular platform for mass audiences. After the game is complete, it will also come out on GOG and I'll most likely release it here as well, but for the duration of Early Access, it will probably just remain on Steam.