Earlier quoted context omitted.
I can answer with the opposite question. Why would you trust your homemade solution instead of heavily invested experts? Auth0 (and Okta and a thousand others) do nothing but auth and customer management and do it at 1000X the scale as most enterprises. They do it way better than your IT team could possibly do it. And support every cutting edge feature and potential use case. You can also get them to absorb indemnity…
Authentication is webdev 101. If you can't roll your own you're in the wrong industry. I really can't think of any good reason to hand crucial control of a site over to any third party, much less user authentication where one breach will potentially cost you millions and land you in jail. The whole business model seems to revolve around being a crutch for people not capable or competent of running their own services.
Because Auth0 and other providers have security experts specializing in prevent hacking attempts and there is no way you can do a better job than them unless you make it your full time job.