Carrier Bag Wastage

Can anyone explain this to the Gnome?

When going to a supermarket, deli or indeed any form of food shop in New York, your purchases are placed inside a plastic bag, which in turn is then placed inside another plastic bag...

Would it not be more sensible, and indeed cost effective, just to make the individual carrier bags strong enough to withstand the rigours of carrying more than one soup can at a time?

Answers on a postcard please...