I would think there might be lots of reasons, but one that comes to mind is that supermarkets are physically designed to get the customer to spend as much time

I would think there might be lots of reasons, but one that comes to mind is that supermarkets are physically designed to get the customer to spend as much time and money as possible in the store as opposed to finding the items on their list(s) as efficiently as possible like a grocery delivery service would want its workers to be able to do.