Because it doesn't happen on a regular basis, and they seem to have reasonable blockers in place to prevent it when they can. But, their drivers are people, and

Because it doesn't happen on a regular basis, and they seem to have reasonable blockers in place to prevent it when they can. But, their drivers are people, and some people will do anything they can to get around that system. If I had other poor experiences with Uber, then I'd be faulting them... but they've done a good job making sure that hasn't happened more than once.