Imagine a world without platform lock-in, where no ban or billionaire could take down your social network. That’s what ActivityPub has planned.
Search indexing is also a huge problem. While it’s possible to create a search index over all content local to a given server, maintaining an index over all content on all servers that you federate is a much bigger issue. And that’s before we get to the problem of migrating associated content when a profile moves from one server to another.