That's not a problem, that's literally the feature. If you don't like it, fork it or move. Can't do that with Twitter.
The problem (at least as far as Mastodon goes) is managing identity between instances. Also, clients still seem to be stuck at the "written by programmers for programmers" state of design, so UX isn't up to par to what a normal user might expect much of the time. The decentralized, federated nature of Mastodon is a big deal to programmers and people who care about censorship resistance/software freedom, etc, but all of that should be completely transparent to the end user. It should be as easy to find, join and leave instances - with a single, common identity - as it is to find and join subreddits.
