Supermarkets come in all different shapes and sizes and a one size fits all policy won't work. For example in supermarket with narrow aisle it makes sense to operate a one way system to assist social distancing. On the other hand there are stores out there with very wide aisles so a one way system doesn't need to be in place.
As for why supermarkets prefer only one shopper per household to visit at a time:
We have a global pandemic at the moment.
Social distancing is part of the Governments guidance to mitigate this.
This means the number of people allowed to enter a supermarket at any one time needs to be limited (meaning people potentially have to queue before being allowed in)
If one adult per shop is allowed in then it means representatives from more 'households' can enter the store at the same time, reducing the queue length outside the store.
As noted upthread there are sometimes very good reasons why more than one member of a household will need to come shopping at the same time. You also need to consider supermarket staff who will appreciate fewer people being in store. It is difficult to staff to enforce this so it is generally a request but it is the right thing to do in the current climate.
As for why supermarkets prefer only one shopper per household to visit at a time:
We have a global pandemic at the moment.
Social distancing is part of the Governments guidance to mitigate this.
This means the number of people allowed to enter a supermarket at any one time needs to be limited (meaning people potentially have to queue before being allowed in)
If one adult per shop is allowed in then it means representatives from more 'households' can enter the store at the same time, reducing the queue length outside the store.
As noted upthread there are sometimes very good reasons why more than one member of a household will need to come shopping at the same time. You also need to consider supermarket staff who will appreciate fewer people being in store. It is difficult to staff to enforce this so it is generally a request but it is the right thing to do in the current climate.