[NT!]

i think part of the reason for tips creeping upward is that, in mid-level or nicer places, really good servers can earn more than that. i don't know what it is - more young people going out to eat, change in restaurant culture, who knows - but eventually they start to be disappointed in 15%. not that they hold it against the customer particularly, but they are always trying to make 20% or better, that's what makes them a good server.

well then eventually the less excellent servers start feeling the same way, and wanting to make more than 15% too. i feel like the concept of waiting tables as a real money-making enterprise kind of rose up more and more in the last few decades, and with it, so did the pressure to tip.

might be totally off as i am pretty young, but just from talking to people who have been in the industry for a while and observing things, that's my guess.