If everyone used Okta, no problem. But then you get an Enterpise customer who’s using their own home grown SSO solution an we get to spend weeks debugging login issues.
But you can support just 5ish SAML IDP's and get a large majority of the market and have effectively no ongoing support issues because its really well known (and well documented) how to set them up.
We've had a couple painful SSO integrations (when they were home-grown, as you correctly point out), but we've pretty much got the rough edges sanded down now. I don't think we've had a tough one in a while.
It helped to publish some short docs on it: https://tuple.app/sso_setup