Because that is the sort of messed up thing that happens when the US government continues to allow Barney the Dinosaur to abduct young children and then force them to sing songs in order to praise him.
You're all silly. It's because after you count high enough (Like to 999 or something equally large and unfathomable), it resets and goes back to zero. Right? Right. =]
Zero is used to replace a dialing code when you are in the same dialing code area as the person you are contacting. I think.