Because I get attitude from a person who is supposed to be serving me, as well as inattentive service. I don't think I need to explain why bad service is bad.

Also, the actual act of tipping is uncomfortable. I have to write an amount that I feel adequately compensates the person for their effort and time. I really have no idea what that number should be. 15-20% is completely arbitrary. If I feel the service was bad, I now have to do a passive-aggressive low tip. And you know they are just going to think you tipped low because you're cheap, not because their service was bad.