This is a really good question. I assume it has something to do with how the game is hosted. I’m guessing Playdek is running their own servers and such. Perhaps Temple Gates is not, and therefore cannot create user accounts/usernames? I really don’t know.
I think TtA has the best system, where you don’t even need to be friends, just start typing someone’s user name and it will point you to the user you’re looking for.
Anyway, if @Strangiato or someone from another developer could enlighten us, I’d appreciate it.