Why do some people not tip servers?

I'm not talking about the "Wow, that was the most god awful service I've ever had at a restaurant. No tip." I'm talking about the, "The restaurant is relatively empty so we received prompt service, our drinks never got below half full, our food was prepared correctly and delivered on time. No tip."

Maybe it's confirmation bias, but I've noticed that several stereotypes about tipping are, for the most part, true. I understand that it's possible for older folks (65+) to not understand that a $2.00 tip for a party of 5 is inadequate... but I find it impossible to believe that a 20-something year old couple doesn't know this.

I served a couple today who left me 48 cents on a $19.52 check. I was standing in the dining area when they sat, so I was at their table in less than 30 seconds to get their drink orders. They split an appetizer and a desert. As they were my only table at the time, they received the best service I could possibly provide. I dropped off their check, walked away from the table towards the kitchen, and when I turned around they had gone. I go to clean up the table and see a nice, crisp, $20 bill laying there to cover their meal of $19.52. I wasn't mad... I just don't understand.

Is there some reason people don't tip servers, or are these people just tightwads who probably shouldn't be eating in restaurants anyway?