system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are tips based on how expensive a meal was?

Why are the tips we give in restaurants relative to how expensive/luxurious my meal was? How much harder is it to carry out a $30 steak from the kitchen compared to a $8 sandwich? I don't get it... Can't tips be a set rate? Or a per person rate?
gpt
At an expensive restaurant you are not just paying for the cost if the food. Part of why you are paying for is food service, ambiance etc. By tipping proportional to the cost of the meal waiters at expensive restaurants will be the ones with the most experience and will be the cream of the crop. 

The other reason is that in fancy restaurants the tip has to be split more ways. The guy going around refilling waters, the people who bus in between courses, the people who bring out new fancy silverware depending on what you ordered, all if those people get some of the tip you leave.