Working for Tips

This is the first of what may or may not be a sporadic feature in which I spend a lot of time asking questions instead of AskJeeves-ing the answer. 

Scene 1: My wife and I go out to a restaurant for dinner. The waiter is courteous, attentive without hovering, refills our drinks as needed, is prompt, polite and does an excellent job.

Scene 2:  My wife and I go out to a restaurant for dinner. The waiter is courteous, attentive without hovering, refills our drinks as needed, is prompt, polite and does an excellent job.

No, there wasn't a trick in there. It's the exact same scenario.  But for some reason the location and cost of food at the restaurant determines what kind of tip I'm supposed to give the waiter despite the fact they did the exact same thing. The Exact. Same. Thing.

But because dinner at Restaurant 1 costs 15% more than dinner at Restaurant 2, the waiter at the first place is supposed to get more than at the second? Why is this? Seriously, someone explain this to me. I get that it's always been done this way, but I want to know why? Where did this idea come from that the cost of the meal is what we should base our tip to the server on and not the quality of the service, especially when two people do the same thing.

And to be clear, I'm not opposed to tipping. Granted, I'd be happier if restaurants would actually pay their wait staff a living wage, ban tipping and reflect that cost in the price of the meal. I realize doing so would mean the end of movies like "It Could Happen To You," but that's a price I'm willing to pay.

And I'm not talking on an individual level here. I'm sure some of you do tip based on service and not price. But from a societal level, why is this standard practice?