In case of OAuth2/OIDC, if I do not use external providers (like Google etc.), I can still deploy one of the many OAuth2/OIDC providers myself and centralise handling of user database this way.
This also means I have one place to support sending last resort emails
As for enterprise auth, a lot of places in fact do not use emails for identity. Sometimes there's more than one login id mapping to one identity (noticeable case - Kerberos/LDAP as done by Active Directory, where your login can come in email-style form and pre-AD form, and the email-style one doesn't have to correspond to an email)
This also means I have one place to support sending last resort emails
As for enterprise auth, a lot of places in fact do not use emails for identity. Sometimes there's more than one login id mapping to one identity (noticeable case - Kerberos/LDAP as done by Active Directory, where your login can come in email-style form and pre-AD form, and the email-style one doesn't have to correspond to an email)