Why is tipping even calculated as a percentage? It's not like the work done by the waiter is suddenly 10x harder or more deserving of reward when they're bringing a plate with a $100 dish rather than a $10 dish.
I suppose that makes sense in certain cases (eg. you getting appetizer + drink + main + desert vs just a main), but utterly broken in others (eg. you getting a $15 main vs a $25 main). If tips were truly based on service it'd be a flat fee per plate or something, that varies per restaurant.
When a restaurant brings a bill that's six times larger than the one to the next table, that's usually because twelve people sit at your table and need service, and two people do at the next.