Or I don't like your real name.
Or your username is cooler.
Or easier to say.
Or I might call you by your real name too.
Like if I've known you for a while.
Or just feel like saying it for a change.
Or I know you prefer me doing so.
But sometimes I just don't care.
To me, a username transcends a person's identity. It becomes them. To me, I am ChocolateFrogs, a person, not someone named after a Harry Potter candy, but the term has changed meaning to be my name. Other names-- Sisen, Disky, Nukaya, Arpy-- are just as unique as a name given at birth, but instead is also one chosen by the bearer to be them.
I know some people don't like being called their username in public, which is totally fine, but it doesn't change that it is part of their identity now.
Of course, that's just my personal view. Maybe you look at me and think I'm crazy for choosing a name after the Harry Potter-verse. Maybe you would much prefer to call people by their real name, because it is perfect and also them. But when I'm out having fun, I have fun with your names too. And that is something we all share.