You probably shouldn't choose any of them, but should set up your own server, but some people don't want to do that, and we don't want to play favorites and say "this is the best server to join" so instead we list a few servers run by our community.
Installing the software is a different use case from using an existing instance. It's simple to do (compared to buying Twitter or writing your own industrial-strength social networking system from scratch) if you are the kind of user that kind of thing is simple for. Otherwise, you have a wide choice of existing instances to choose from.
There are many more than seven existing servers. Consumer choice is a good thing. User confusion less so.
