Nostr offers this (completely portable identity) without centralizing anything, that's the selling point. Similar to cryptocurrency, your Nostr account is tied to a key. If you possess that key, you can connect to any Nostr node (assuming you meet their criteria - some require payment, some might have banned you, etc) and post to your account. This makes individual Nostr servers wayyyyy less important than servers are on fedi, and cuts fedi style inter-instance drama off at the knees, because servers are really just conduits for posts to flow. Yes they can differentiate themselves on e.g. uptime or performance or spam filtering, but at the end of the day they are just a conduit, not a community like fedi servers can be and often are.

Right now on fedi it's a pain in the ass to move accounts to a different server, if the one you are on supports it at all. Identity is only poorly portable at best. Mastodon only kinda sorta supports it in a janky way and there are plenty of bugs. Whereas on Nostr, which servers carry your posts doesn't even matter nearly as much as it does on fedi, where it matters a lot. It doesn't matter 0% on Nostr, but the importance is vastly de-emphasized.

It's just different approaches, with different advantages and disadvantages. Censorship resistance from the perspective of the user is a much more emphasized goal in Nostr's design.