Because restaurant owners are cheap bastards that would sell your kidneys for a dollar if they could get away with it. They wouldn’t pay 20% of food prices, but they would pocket the 30% increase of food prices to “make up” for having to pay their servers

Servers are all paid at least minimum wage like a million other workers. Are you going to start tipping all minimum wage workers

Another factor is that serving is somewhat entertainment. People want the server to be engaging and personable, disappear discretely, and return at the slightest whim. It feeds into individualism and quite frankly, the experience of being cared for in a way that only the elite receive on a daily basis. This is why servers cannot be compared to teachers, etc.