It's to make it less likely that the username is already in use. We could use email for the username in those cases, but email is not always available. In the past we didn't have a way to allow the user to change the username if there was a conflict and instead the first login would just fail. With the introduction of first time social flows we could improve on this.
We could allow selecting the strategy to use. Then allow the user to change if there's a conflict. We already allow users to change email if there's a conflict so can do the same for username.