There's a lot more to that. A bank doesn't want the "back" button to work forever; they want to control the lifetime of your session, ideally on the server. Google wants to let you sign into multiple accounts on the same origin. Many others want to have seamless single sign-on across several of their web properties. Sometimes, you want the change of your password to invalidate other sessions (say, when recovering a compromised account); other times, you don't want to kick out your smart thermostat and have to set it up from scratch.

Admittedly, there are some simple use cases where HTTP auth is all you need, but it's just way too inflexible, unless you turn it into some mammoth spec that is never going to be as flexible and tempting as managing user identity yourself.

Especially since HTTP auth doesn't actually mean you can stop doing that anyway. You're still handling account creation, password checking, all the abuse / bot detection bits... all you're getting rid of is the sign-on and logout functionality, which is really not that complicated to begin with.