I rarely get useful assistance in large chain stores. Asking "where is the pesto?" usually gets an answer like "I dunno, somewhere in aisle 7 maybe" with an indifferent shrug. I'd happily make three attempts at using a clunky voice interface if I got the answer "Aisle 7, bay 11, shelves 3 and 4. We have 18 different kinds of pesto currently in stock. Would you like me to take you there?"
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.