This is just flat-out untrue, OIDC or SAML plus SCIM should be the default for any enterprise-focused service provider or "you're doing it wrong". You can offer your own IDP as the default, but all of the problems that need to be solved to allow your customers to configure their own IDP are important to the design/architecture of your service and the only reason these providers are treating it as special is because they didn't build the integration between their service and their IDP correctly the first time. Provisioning and authentication are critical to security and you're actively harming your customers if you require them to use your own IDP solution in order to use your service.

As a volunteer at a volunteer run non-profit, I agree! Nobody makes any more at the org, and it would be great to have SSO for things without having to pay more 150% of our total annual budget to get it...