This is an implementation issue, not a spec issue. Pages can delegate to another identity provider (primarily used for vanity URLs, for example I could say that

This is an implementation issue, not a spec issue. Pages can delegate to another identity provider (primarily used for vanity URLs, for example I could say that https://nullable.se/ https://nullable.se/ should map to my Google OpenID, for example), and the identity provider can claim a different URL than you asked for (for example, Steam always uses https://steamcommunity.com/openid https://steamcommunity.com/openid as the provider, but will claim a different URL for each user). Combine these, and Google would have been perfectly capable of making everyone just type google.com. That they didnt' says more about Google than OpenID. Besides, people were perfectly capable of showing the same pickers back then as you are forced to use with the current OIDC/OAuth2 abomination.