Just a few reasons:
- People would change their names too much, and it would become very confusing to figure out who is who.Trying to talk to Ludicolo99, then realizing that marshtompmaster33 is the same person can get very aggravating.
- People will not always pick appropriate names. Sexmasterchildrapist wouldn't go well here.
- People could do stuff to break rules, then later change their names to make them harder for people to report and deal with.
- People can change their names to impersonate others, and since some characters would not be visible, you can lead to a lot of trouble with doppelgangers.
There are several others, but this gives you the basic idea.