Choice. In reality, you'd likely have only one or 2 viable app stores. Most developers won't waste time with 5-10. Most people will choose 1, a small number of people will chose a second one possibly, and very few will use the others. However when Google Bans one app, that developer can go to another store. You might prefer app store #3's interface or they might be better at showing you the content you want to see. Same way a lot of people prefer 3rd party twitter apps for example.

The internet is wide open, yet we only have a few browsers that are really popular. Social media is a bit more diverse yet there are a few programs that are the big ones depending on what type of content you are trying to post.