If you can afford a big tip, and social mechanisms shame you into giving it, then great! The restaurant makes a lot of profit.
If you can't, then the restaurant can still make a small profit off serving you anyway.
As for why it'd increase over time... maybe that's just an effect of people getting a bit richer, but the marginal cost of serving a meal not so much?
You could argue that, but only by ignoring practical matters of effort/level of service/value that attends a different price.
I'm 40+, and when I was in elementary school, pretty much every source I encountered was consistent with 15% as the norm, 20% as a reward for exceptional service, 10% as a signal for tolerable but substandard service, and anything lower as a signal of “abysmal service and I'm only tipping to let you know that I did not forget about the tip to avoid ambiguity.”