All the rest of you pig-fuckers pay cash. And people are selfish. This is not a trick question. Yours do, of course. Yours always do.
Selfishness is a biologically adapted trait.
Let's pretend there are two Cro-Magnons huddled together in their Ice Age shelter. They're both starving, and there's only enough food for one. Now, one of the Cro-Mags was born with the Selfish gene, and the other inherited the Benevolent gene. The Benevolent Cro-Mag gives his food to the Selfish one. The Selfish Cro-Mag eats.