I'm guessing it's because the OP wants riders to also be drivers, so you don't end up with one set of people making less than minimum wage to drive another set of people around (the Uber model). Of course, this won't work with formal "credits," since people will just create a market exchanging "credits" for dollars. This is where an informal reputation system shines: people eventually identify freeloaders, and kick them out of the carpool.

Because when it's money the goal is money. I desire the goal to be efficient and fair ride sharing.