...for many reasons!
It would take forever to go over all of them, so...I'll just give an example of what happened at work the other day. There was a guy who ordered a large soda, so, I gave it to him. Then after drinking part of it, he asks how much it is, despite there being a GIANT menu sign behind the counter, and so I tell him, and he says "Nah, that's too much."
So...he dug through the tip jar for the cash, handed it to me, then left. I was so stunned that I didn't even say anything...and he didn't even give me enough to pay for the thing. (Sales tax...)
Why are people such assholes?