why are usernames permanent?

[Cable]

For basically everything account based on the internet, the username is always permanent

Why cant we just get a random ID number to identify the account and change the username whenever we feel the old one gets weird? (as numbers never really age, when used as an identifier, you can change the display name to whatever you feel represents you at the time)

I am curious because looking back at since i began using usernames (13 years of age and registering for accounts on various MMORPGs), I've thought up a lot of what I would now consider "not my style" (for example... "dragonslayer9997", or "wingwarrior069")

It's only a problem I've made for myself now since I'm no longer "feelin" the username i used for the past year and a half

Most of the time if you cant delete the account completely or change the accounts e-mail, it is impossible to change a username for yourself

So why is the username impossible to change?
^^^(as numbers never really age, when used as an identifier, you can change the display name to whatever you feel represents you at the time without the hassle of not knowing who the username belongs to)