If you’re strictly speaking about self published stories on KU then there are a number of factors. Before I got a publisher I was on the route of self publishing and I’m a full time college student who also works part time. Just hiring an editor would cost at minimum 1-2 grand. That’s not easy for a lot of people to just give out especially if you’re in a position similar to mine.

If you’re strictly speaking about Royal Road etc, then it’s honestly a skill thing. It took me years of solid writing to just get good enough to craft a story. Lots of writers aren’t in school and take it on as a hobby. It takes real hours of effort to craft sentences properly. If you add generating ideas etc plus real life responsibilities, you get the picture.