Centralizing platforms aren't necessarily seamless either. Having the same general contents propagating around, but different sites to seeing them, allows for creating the ideal experience to a given user, and each user expecting different things for an ideal experience.

Like in my case, I'm in Mbin, which would look like the least feature-complete of the fediverse softwares at first glance. But besides actually having what it takes to make a functional forum, it can bring together microblog and thread posts, and the lack of visual noise in the UI while the UI also being overall text-oriented help me focus. Plus it's very responsive to scripts when a feature isn't natively available. And I can connect to other sites thanks to RSS-tracking and protocol-bridging bots.

And from it, jumping to other forums such Twitter, Retropie and GOG's becomes seamless too.

For me, that is an example of seamlessness.