Ask HN: How many of you are rolling your own auth?

There's several auth-as-a-service providers on the market right now (Auth0,Firebase,etc). (By auth I mean, all the systems that facilitate password hashing and user authentication, along with SSO integrations.) Their common marketing argument, don't reinvent the wheel, and if you try, you'll probably get it wrong. This is pretty compelling as getting auth "perfect" seems to require some decent research and understanding. However, when viewing the customer pages of some of these providers, I don't see a ton of companies that I'm familiar with. Out of all the sites and tech tools I use, are they rolling their own auth, all of them? Once a site gets big, does it just get too hard to scale or adapt to the third-party provider?