Not to mention there’s a whole lot of middle ground between writing your own auth logic and using a hosted service. Any reasonably popular web-oriented language has plenty of fully featured, high quality libraries you can often just drop in.

Hell, we use Laravel at work, which includes pretty robust auth features out of the box. It feels like such table stakes stuff for a web framework that I find the idea of paying for a third-party cloud service absolutely bananas.

It also includes pretty solid notification and payment solutions too, though these usually plug into services like SNS and Stripe and don’t really fit the “self hosted” bill.