I read an argument once that that tipping is probably illegal under US employment law because, in practice, its racist. Research indicates that servers who are ethnic minorities are tipped less than white women, who are tipped most. Sure the employer isn't doing the discrimination themselves but they are setting up compensation scheme that, in practice, is discriminatory. This is called "adverse impact" in US employm…
It seems like you're only classifying the physical labor as "work", and ignoring the skill and effort it takes to do all the stuff a waiter/waitress does. There's a lot of memorization involved, being able to answer a lot of different questions, making sure the meal gets delivered as soon as possible, with the right stuff, and checking back at the correct times to see if anything else is needed, including drinks. It'…
Even if "it's a lot harder than it seems" (which really, what?) it's not 90% more work the busboy was doing... remember the busboy also buses the tables. Answering a few questions about the menu is not significantly more difficult than clearing tables.
(Then there's the places where the kitchen staff brings out the food...or they have "runners" who are responsible for bringing out the food.... really, I hate to generalize but as a group servers have a seriously inflated sense of importance, restaurant service is a team effort)