It's not a marketplace, it's a lottery for developers.
Quite true and one where you have to make something that likely already exists.
Besides games, I haven't downloaded a new app in a long time. Most of the ones I have and use are mature and do their function well. There was a wild time when many functions and app types weren't yet developed but now? Not everything has been invented, but most categories are fairly mature and there is less and less room for groundbreaking apps for the overall market (still plenty for niche markets).