On a community I’m active on you can change your username but the policy as far as I know still stands as: “Username changes are allowed once every two years.”
That way you can still change your username, perhaps not as often as a user would like but it means less work but it still doesn’t stop users asking how / who do they contact to change their username.
My opinion is that if everyone could change their username all the time it would become confusing to remember who is who lets say today you are @jenmck and tomorrow you can @Ilovecats7128 and 5 days later its something else its just confusing.