The basic business model of a restaurant is repeat food sales. But some criminals are opportunists who are not worried about repeatability; they heavily discount the future in the face of a sufficiently attractive short-term payoff.
That's true, but the majority of taxi/ridesharing drivers, and the vast majority of honest well-behaved drivers, will want to be in the business for an extended period of time. There are various methods to pre-filter potential business partners by estimating their honesty. Reputation systems and signaling are two broad methods which I think can be adapted to the ridesharing sector.
I do not share your optimisim about reputation systems. One of the benefits of an employer (who can be regulated) is that they have an economic incentive to do that filtering on behalf of the customer in order to protect their own bottom line. I don't want to spend time figuring out if a cab driver is legit or not, life's too damn short for that. That's part of what I am paying for when I hire a cab.