It's much more lucrative for authentication/authorization/user management SaaS companies to build trust with their customers than to exploit that trust.

Furthermore, with the economies of scale, there can be more investment done on security & protection of users by a common element. Think of it as a collection of companies pooling their resources on a single engineering team which is responsible for building a rock solid authentication system with many useful features. This helps little guys build on top of something which has the same efficacy as auth services that large companies can afford to build.

At the end of the day, it's a trade off.