An Uber can't pick up a new passenger without cancelling their current pickup or picking up their current passenger and finishing the ride. Either way, the passenger both can see if they're coming to pick them up, and are made aware immediately if the driver cancels.

With cabs, you're expecting a cabby to call to dispatch that they're not picking you up. A cabby can easily be on the way to pick you up, find someone on the way and never let you know. That's a big difference in both what's possible, and what the reality is: cabbies never let you know if they're not coming in my experience. That said, many cab companies have an app now.