The problem is that network architecture of Nostr is different from Twitter (and from Mastodon, for that matter). One of the core ideas of the Nostr protocol is

The problem is that network architecture of Nostr is different from Twitter (and from Mastodon, for that matter). One of the core ideas of the Nostr protocol is getting rid of the idea of "logging in" or "creating an account" on any given relay, and as such it needs some way to reliably identify which posts were made by a particular user. That's where PKI comes into play: it's not layered on top of a login system, it's a complete replacement.