Wow, after thinking about it, I guess it could go like this: driver gets a ping, accepts it and quickly takes a screenshot of the order then canceled the pickup, races over to the restaurant and snatches the food, if questioned they show the restaurant the screenshot of the order and get away with a free meal. I hate thieves.
Considering that’s the case, what if the order is marked ready, the thief who cancelled picks up the order. Now the customer is waiting while previous drivers marked it as already picked up. Uber will immediately assume it was the first driver that cancelled was the thief.
Also they contact the restaurants, so they will become suspicious immediately.