And what happens when a passenger makes unreasonable demands - "Hey, I need to stop here and do some shopping, so you just wait here, and BTW, I only have half the money, so you won't get paid what I agreed to pay." - and then threatens to give the driver a bad rating if they complain about it, what mechanism is there to address that deliberate libel once it's posted publicly? Sure, the driver can give the passenger a bad rating, too, but that won't help him any.
And that is the failing of relying solely on a reputation system. There's no protection against liars.