Depending on how they manage their database of user accounts, usernames are probably a unique identifier. The system may depend on each user having a single, unique, unchanging username.

It is also probably to prevent exploits and impersonations. Someone could choose a similar name and the same profile picture and pretend to be another user. Potentially most destructively, a mod.

Finally it can be annoying if users change their names all the time. It makes conversations hard to follow. And again, it might be used for exploits like evading bans.