They have more of an opportunity to prevent carjackings than the drivers do. For example, they could require enough information to create an account that if you stole a car using your account you'd be immediately identified and reported to the police by name.
The downside of this is that it decreases Uber's revenue. If Uber realizes the benefits of low security in their app, but not the costs, they have to incentive to take these or any other precautions. Making Uber responsible for the negative externalities of its service means that they have to take them into account when determining how to maximize profit.