Cause they're always on their phones so they literally never have to BRB. The few of them who still use computers will just open Discord on their phones and continue the conversation if they have to step away from the keyboard for any reason.
I feel like BRB for me was when I was in an AIM chatroom and my mom had to make a phone call, so I'd have to get logged off. And I'd say BRB but would inevitably never be B.