Because what originally started out as a reward for exceptional service has become obligatory. Worse, servers universally decided about five years ago that cheap was 20% and a good tip was 30% (instead of 15% and 25%)... right about the same time it became chic to order off the wall instead of getting table service. I've also noticed that service has declined in general and surliness has increased - all while feeling entitled to more money. The rest of the world doesn't do it this way. Servers get a living wage and if they deliver exceptional service they get a small gratuity. I don't begrudge anyone making a living, but the system is bullshit. Tips also provide a marvelous way for the restaurant to skim off the wages of the employees - so you get pissed off paying $2 for someone to hand you a Jack Daniels on the rocks and the server gets pissed off because of that $2 they're only making $125. The whole system is bullshit.