The main issue is spiky demand, you’d have keep a cadre of employees around to minimize peak latency. Offloading tasks to the shopper scales well with usage.

Also revenue loss due to fewer impulse purchases. You could still have candy bars in the line to get to the counter, but it's not the same thing as merchandising in aisles and on end caps.

With robots doing the picking and packing the employee problem becomes reduced, but it might take some serious innovation to reliably get customers to leave with more products than they went to the store to buy.