Because customer service is draining. Because even when you get paid $75k, dealing with assholes still sucks.
I'm gonna hand wave some more: most Americans when they think of good service beyond just being quick on delivery. Good service is about having a friendly conversation and building rapport with another person. It's may be weird for those of us with introverted tendencies, but this is Middle America with big smiling teeth.
Now.. by giving me an immediate incentive (tip) to smile through a bad customer's rudeness (or even harassment--please don't ever hit on people paid to serve you, ever), I'll continue to smile and pour on that syrupy niceness even when I just want to tell a person that they're an asshole and never have to deal with them again.
Personally, I have absolute no problem with tipping, or paying higher prices to pay restaurant workers a better wage, but I think people who are pushing no-tip don't understand the complexities in place.
In the restaurant world, at the end of each night, you'd get a huge wad of cash, and you felt you had worked for it (even though it can be argued people tip consistently). So even though the carrot may not be real, it still convinced people to act a certain way.
Now take away that carrot, take away the cash. You just have better paid McDonald's workers.