There was also some very good and valid reasons why real people wound up with those usernames - mainly, that the signup process (from the App I think? maybe also in New Reddit?) both downplayed, and obstructed changing, the default username during the process - and instead led the user to believe that only the "display name" selected later would appear to other users on the site.
Completely omitting the fact that anyone on old reddit or accessing through an app would only see the username, as "display names" don't seem to have ever been served via the API.
To many of those users, they had no clue that what people were seeing attached to their comments or submissions was "extravagant_mustard_924" and not "Cool Dude Brian" or whatever they'd put in as their display name. They were led to believe that the latter was all that would display, and that signing up with a default account name would only determine what they entered in the top box while logging in.