We can, passkeys are being adopted all over the web. If you specifically mean for Lemmy or fediverse services, it’s probably just a matter of adding support. It isn’t hard, per se, but it is important to get it right.

You can store passkeys in a password manager like BitWarden and they become portable. Then it doesn’t matter if you have a centralized authentication server. You just get logged in with your passkey, supplied by your password manager.