I haven't spent almost any time in Europe, but in the US at some point it seems to have gone from a setup to incentivize that "soft skills, fast response, engagement" to one that I don't feel does that much due to the social obligation. When your worst case scenario is 18% and your best case is likely 20% + rounded up, who bothers to change their behavior? In my experience it's more like a commission than a tip, and that's because the only real impact is it encourages the waiters to remember to promote expensive alcoholic drinks before and during the meal, since you can earn way more money getting 18% from a table who orders 2 drinks than you do by earning 21% from a table drinking water. As a commission, it should be paid by the employer.
I call tipping the "A*hole discount" because only someone who is comfortable being seen as one would consider tipping 0% or even 10%. And servers will tell you that if you receive terrible, horrible, very bad service you should never tip zero, you should speak to the manager instead (since yadda yadda, the tips are for everyone, you don't know whose fault it is, etc). So instead of tips being a way to reward good service, it's actually just a discount reserved for people who are so uncouth that (A) they don't care if everyone at their table thinks they're an ass, and (B) they don't mind taking food off the table of low-wage workers (very low in states that have the horrible policy of a lower "tipped" min wage). Do those people really deserve a discount?