There are some studies that found that tipping isn't necessarily related to quality of service

Edit: interesting to see down votes. Here's an excerpt from academic research; "other studies have failed to find a significant relationship between tipping and service evaluations (Bodvarsson & Gibson, 1994; Crusco & Wetzel, 1984; Lynn, 1988; Lynn & Latane, 1984; May, 1978)"

https://www.google.com/url?q=https://ecommons.cornell.edu/bitstream/handle/1813/72354/Lynn46_Tipping.pdf%3Fsequence%3D1&sa=U&ved=2ahUKEwib86qX76T8AhUpK0QIHSohC1UQFnoECAYQAg&usg=AOvVaw3nkAIqOaefKJ32GoQqU7a7

And from the Freakanomics podcast:

"Lynn’s research shows that tipping is an unfair way for workers to be paid, because personal characteristics like a server’s race, gender, and appearance factor too much into customers’ tipping decisions."

https://freakonomics.com/podcast/why-does-tipping-still-exist-ep-396/