Because tipping is racist, sexist, and classist --- tips don't correlate with levels of service and reduce predictability in working and using a service.
One reason why Uber has been great for the black community is that hailing a cab has traditionally been more difficult. Studies show that Black patrons get lower levels of service and black waiters get lower tips than their white peers (because of enduring stereotypes).
> Sexist
Women are often tipped more based off of looks than their male peers. Similarly, warding off advances and harassment is a thing.
> Classist
A diner and a 5 star restaurant often have competing levels of service, yet % based tipping means that the restaurant servers take home much less more money. Similarly, things like crooked teeth can affect how your patrons regard your smile.
Surely this has to be facetious? If not I would love to hear you elaborate?