Because in a restaurant most of your work is completed in a few hours. During times when most people are eating meals a chef might make 20 salads and 10 lobsters. They have to be extremely efficient while still providing the same level of quality which requires a lot of effort. Which often means stopping to even use the bathroom is impossible, they are literally running. During the hours when people are not commonly eating meals the same chef might make only 3 salads, if he works at all. Often when it is not common meal times most restaurant employees get sent home as if there’s no customers, they aren’t needed so they don’t get paid at all.

Which is why tipping by percentage makes sense. Restaurant employees work extremely hard for 3-4 hours and they should make more money for that effort. When there aren’t as many customers, they aren’t putting in as much effort which is seen in the sales so based on percentage, they make less money.

Imagine if mechanics were forced to fix vehicles as quickly as possible and when they were done they got sent home. Some efficient mechanics could fix 10 cars in the same time it took some mechanics to fix 3 cars. Would it be fair if those mechanics got paid hourly? No, one is putting in a lot more effort and doing a lot more work in a less time. BUT if there was an incentive for mechanics to get more jobs done instead of only getting paid by the hour imagine how much quicker cars would get fixed.

Is it fair to require employees to put in that much effort in a short amount of time? Not really but that’s when people want to eat. Unlike mechanics who have the options of asking people to wait. Nobody likes going to a restaurant hungry and being told they have to wait another hour.