whats wrong with all the groups growing equally as fast?
That's not how money flow works. Money yields more money, there's no real way of circumventing that outside of socialism.
Maybe if every single dish in a restaraunt was the same price. But you can't judge "classiness" of a restaurant based off the price of a dish, because they can vary immensely in price
Sure, but tipping based on percentage is supposed to be a rule of thumb. You can assume that in a party of 5 or so that between everyone's order, the average price of each person's meal is approximately the average price of the menu items. Since the average price of menu items scales with restaurant (not necessarily food) quality, and since higher tip should be paid to wait staff at higher quality restaurants, it makes sense to pay tip based on the cost of the check. Otherwise it'd be difficult to figure out how much to tip-- you may tip $2 per person at Red Robin, but that same tip isn't going to work at a place with hundred dollar plates. It's simply a tool people use to calculate about how much it makes sense to tip.