Most of the restaurants I know earn more than managers. While waiters are categorically underpaid, the living wage a waiter can earn, at least in the United States, generally comes from tips. Yes, this is a problem for people who work as waiters. And yes, if restaurants and bars want to keep great employees for the long term, they should pay them much better.
That said, if you're a waiter, learning to increase your chances of getting good tips is a good plan.