There's a single URL for OIDC IdPs too. It's not supported in every web app for the same reason SAML isn't: it's a two-way exchange to set up, not a one-way exchange. In OIDC (OAuth 2) you need to register a client first. In SAML you need to register a Service Provider first. Only after you've done those steps will you be able to plug the magic configuration in and have it work.

SAML is too limited to compete with OIDC/OAuth2 on the web, but it's got some advantages for enterprise authentication, so it won't be going anywhere.