However, usernames have to be displayed, shared, searched, etc. It could in theory be possible to encrypt this, but with the requirement that it be two way (so as to allow display), it is more trouble than it is worth, with minimal security gained. Most security researchers would probably also point out that if you are able to get to a point where you are dumping a database of user credentials and emails, you are probably also going to be able to retrieve the encryption key, rendering all of that encryption moot.
I've done similar things and it makes life hard. People need a way to refer to specific accounts, to talk about them, to troubleshoot. And for most sites your username is a public identifier, a way for other people to talk about you.
But look at the way AWS credentials work. They're generated for you by the system, and no-one refers to someone else's login. But they still, very deliberately, split your AWS key into a public part and a secret part. Because it's very useful to have a public identity for a credential.