I didn't say it was impossible, just impractical and that is why majority of services that use SSO only support google, apple, twitter or facebook. You write the code specific to these few providers once and are done with it for good. There is little reason to invest time and money for adding generic support for other providers. It's just the way it is. If OIDC protocol would get streamlined a bit, we could easily have universal support. But then again, these big providers would likely be stuck in the current version and not bother adjusting to the new, simpler version, if it would come to be.