It absolutely could, but then you're giving ownership to whoever manages that authentication service. Since the creation of your username is what creates your k

It absolutely could, but then you're giving ownership to whoever manages that authentication service. Since the creation of your username is what creates your key pair, you own your encryption keys. And since your username is anchored to the bitcoin blockchain, there is no central authority that can say "you no longer have access to this account." Logging in with Google or Facebook or any other third party service is probably the biggest problem with traditional apps. Perhaps more so than them storing your data. Sure, they can (and have sold user data), but if these providers are the owners of your digital identity, they also control whether or not you have access. Slack, Twitter, Google, et al have been known to lock people out of their accounts without warning.