Honestly, I’m shocked there aren’t way more. We have the Google Store, Samsung Store, Amazon store and…that’s it?

There are plenty of repositories for F-Droid, and from what I’ve heard, also for Aptoide.

That said, it is not really beneficial for most companies to compete with Google Play since they know the user base will be smaller, user experience will be worse (install warnings, no auto-update), and people may get affected by malware if they don’t pay attention to where they are downloading things from (may download a scam app directly instead of the legit app store).