That’s… mostly because of popularity and it depends on whether some service is offering OAuth and if the website in question is using THAT identity provider.

For example, mastodon is technically offering it.

github.com/mastodon/mastodon/pull/16221

but this is the docs page:

docs.joinmastodon.org/admin/optional/sso/

So the answer in this case is to just grow, promote and support what we’re already doing: fediverse stuff.