This service is dealing with stuff like OAuth, SAML, two factor, logging, monitoring, scaling and apparently even does authorization (from skimming) and more.
In other words, tons of stuff that many businesses don't need or want.
Google, an indie game, a SaaS, or a custom web app shop all have different security engineering requirements, including authentication, often per project.
Also outsourcing auth and not having full control over it is not feasible or even allowed for some domains or projects for a multitude of reasons. Not to mention that using an external service has at least a constant complexity cost.
That said, these kind of services are definitely worth considering for many. There is something to be said about advantages of specialization and cost-benefit as well. Reliability is not optional for an auth system, and I'm sure these engineers are really good at what they do. However the challenge would rather be convincing business, not engineering.