It almost seems too obvious to say, but computers have different strengths and weaknesses to humans.
There are also social factors involved that may be advantageous to machines. Self-checkout machines offer an objectively worse experience most of the time, but a lot of shoppers prefer them. Some customers don't want to interact with a human cashier. Some will be buying embarrassing items and prefer the apparent anonymity of self-checkout. There's also a subtlety in how the self-checkout machines are arranged - because there's usually a common queue for several machines, customers don't feel pressured to finish their transaction quickly.
Shoppers may prefer being assisted by a robot for a variety of reasons, even if the robot isn't quite as good as the average human worker.