If SSO were so easy to solve we wouldn't have a gazillion companies for it.
It's probably easy enough if you are a really good engineer, but like 90% working in this industry aren't.
Also you ever implemented OAuth2 or shudder SAML? Not how I would like to spend the one life I have been given.
I think the fact that there are a gazillion companies for it, and they don't compete on security, but instead compete for billboard space in the Mission District of SF and Redwood City California, shows how easy it is to solve.
