[user5994461]

Because otherwise anyone could make an authentication provider that's authenticating as [e-mail] and assume your identity.

Remember that the goal is to delegate authentication, user profile and/or user creation. It's implicitly trusting everything coming from the third party. For instance when supporting google login, it's expected that google only validate user accounts that really exist and are hosted by google.

[icebraining]

Well, no, it would only accept IDs of the format *@yoursite.com for the provider yoursite.com (or the provider yoursite.com has delegated to).

[user5994461]

Authentication cannot be bound to any email or format.

For instance when using google authentication, the user and the email can be anything, because google apps support custom domains for paying customers.

If you want to rely on emails, just use emails.

[icebraining]

For instance when using google authentication, the user and the email can be anything, because google apps support custom domains for paying customers.

That's what delegation is for.