SMTP survives in its federated form because of path dependency. And people didn't start using email because it was federated, they started using it because it was an entirely new capability to most people. The federation was how its creators managed to implement such a thing at that period of time, but nobody in the 90s was saying "email is great because it is decentralized!", they were saying "email is great because I don't have to wait days for a piece of paper to travel across the continent and it's cheaper and faster to type than to make a phone call!".

Another commenter put this well: decentralization / federation of social networks thus far seems to be a feature but not a benefit. What is the benefit to users, what can they do with a decentralized network that they couldn't otherwise do? So far, the answers to this are ideological, the benefit is "I don't want to use an application centrally run by XYZ corporation, and this lets me achieve that goal". I think that's incredibly reasonable, but it is downstream of ideology rather than utility.