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.

"To send email, first set up your own server."