We could borrow some OIDC stuff, such as the "/userinfo" endpoint. We didn't because we already have atproto endpoint to achieve this, and because we want to keep the requirements needed to implement a PDS as light as possibe.

Since a "federated OAuth flavor with user IDs that can switch authority" (as is the case of did:plc), is inherently incompatible with OIDC (in which a client has to know, and trust, the authorities in advance), we didn't want to tie our specification with OIDC to avoid misleading devs.