[fragmede]

Well, you don't need to sign up for yet another service for one. People don't want to have yet another email and password to remember. That's what OAuth and all that machinery was invented to solve.

AuthN is one of the few use cases where having a public shared database/blockchain and users authenticate using their private keys works out better in some respects because the alternative is to depend on a centralized user database owned by a private company (eg Facebook). What happens on all of the sites you used Facebook login on, if your Facebook account gets suspended?