i may be the only person who feels this way, but this is my take on tipping waiters and waitresses:
to me, it's a job i would never like to do. and the reason for it, is because i personally find it to be demeaning. i in no way want to offend anyone who performs those jobs. i have respect for anyone who gets off their asses and works for a living. but i would personally not like to spend eight hours a day waiting on people and trying to please them as much as i can simply to get a tip. for that reason, i enjoy giving them good tips, and i appreciate their service.
in other jobs that people in this thread have brought up, and why they wonder why they don't get tips, it's because those other jobs are standard. as in, the business does their job their way, and you either take it or you don't, and go someplace else. in a restaurant environment, you are expected to cater to the customer's needs in a way where you're essentially waiting on the people hand-and-foot. and you also have to deal with sometimes terrible attitudes, and are expected to accept it. we've all probably seen the unreasonable and downright angry customer who complains about their steak not being cooked to their satisfaction and causing a scene. if you deal with shit like that on a regular basis, you deserve a good tip for dealing with people like that.
my two cents.