It's really weird to me how recommended tip levels have shifted in my lifetime (really in the past 15 years):
When I was a kid in the 80s/90s, 15% was standard (there was even the mnemonic to calculate it: 10% is easy to calculate, then take half that and add it). 18% was for really outstanding service, 20% was for outstanding service when someone wanted to show off, and 25% was basically unheard of besides some gossip mag detailing how rich a celebrity was ("They tipped 25%!")
Now I see tip amounts that default to 20%. My hypothesis, and perhaps this is a relatively sad commentary in the US, is that the level of inequality in major US cities is off the charts: for many people eating at a nice restaurant in a major city, food, even eating out, is a tiny portion of their overall budget, so adding 5% is really nothing, but people working in restaurants are really struggling to get by (and diners know it), so the increased tip rate is basically a consequence of trying to make unaffordable large cities bearable for workers.