Cognito and Auth0 are so popular because 1) somewhere in the past 5 years developers got bullied into believing that doing authentication is now hard, high risk, dangerous, hard to get right, and all kinds of scare words in the name of the security theater, and, 2) there are a lot of incompetent people who somehow don't understand that you should not put your database on the public internet without authentication.
Just outsource all of that to us, we will all take care of all that very hard work for you, really, just 5.99$/month at first, and when we inevitably get hacked, because actually it's us that have no fucking idea what security is, since we only understand security theater compliance language, you can point your fingers at us so you don't get fired, and we will wash our hands with some vague PR words. Win win.