For 99.9% of businesses, don't roll your own auth. Just don't do it. Use an auth service.
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.
The risk of your home-rolled auth, with all the edge cases and requirements that go with it, vs. outsourcing to a company who's entire reputation relies on doing it properly with an army of experts, running on enterprise-grade architecture, with round the clock support/updates/ops?
Are you seriously saying that you can roll your own auth better than Auth0, Fusion etc. can do? And that the hours startups spend on setting it all up vs. an afternoon wiring in an auth-as-a-service provider is time well spent?
If you're a small company, don't risk it, don't invest time in it, just buy it it and plug it in and know that it's all enterprise-grade under the hood and you're covered (far more than you could ever afford to be).
If you're a big company, and you've done the maths and figure that the risk/effort vs. expense is worth it, fine.