Have you seen the > 150 entries in Search results for '#theme' - Discourse Meta? Many of those offer significant changes in functionality. There are only ~40 items in Search results for '#theme-full' - Discourse Meta, and some of those do make some rather significant changes to how things look.
One answer for where there aren’t 100s of themes is that there isn’t a marketplace for themes, I think because many developers are big open source people. In some sense, this is a blessing since it doesn’t give zillions of people the desire to create trivial themes on the off chance that someone will pay them for them.
Mostly, if there’s something that you know you want, you can ask in marketplace.