I remember talking to the Cognito team about password reset and arguing with them that being able to set a password was a required feature. They were like "no, why would you never have to not go through the reset flow? That's a security problem." Then of course they added it in a few weeks later because every admin needs to do that. So at some point they had a bunch of people working on it who had like zero operational experience.

Two benefits to Cognito are (1) it allows you to log into a service without having any credentials locally, and (2) that Cognito identity allows you to provide access to AWS resources. You can probably do that now, but plenty of solutions still require an on-device key...which is an obvious security issue.

Also, using your own backend for authentication made it easier to manage things because your auth wasn't trapped inside Cognito.