Why are Ubers allowed to cancel even though they’re at your location?

This is the second time this has happened to me lol. I ordered an uber to work and as soon as the guy arrives, he cancels moments later. To be fair, I wasn’t outside once his arrival because I was handling my dog. But I came out not even three minutes past his arrival. Usually there’s a few minutes of grace or a text to let your passenger know you’ve arrived. In another situation I once had someone drive PAST me twice before canceling. Then uber gets me another driver that’s twenty minutes away. It’s so frustrating especially when I go from arriving early to work to now being potentially twenty minutes late. Can anyone explain why this happens? Are they in a rush? If I were to cancel an uber that was even three minutes away I’d have to pay a fee (rightfully).