Because implementing that shit is hard.
Nostr definitely has some interesting/good implementation details, and on a spectrum of ease of censoring users, Bluesky is on one end, Fedi is in the middle, and Nostr (if it gets big enough) is on the other end.
But also with Nostr, I really hope you backed up those account keys, because if you didn't and you lose your device, your access to that account is gone forever.
Every way of decentralizing a social media system has advantages and disadvantages.