Thats because they r trained to pack different things into
different bags. Like u buy a bottle of shampoo together w sme
canned foods. U will find they put ur shampoo in a small plastic
bag, then put this small plastic bag in e larger bag that contains
all e canned foods. U can always tell them to pack together, dun
need any extra bags.
Thats a different story if u go to a chinese style supermarket/
mini mart. They won't separate ur merchandise. They will dump all e
things in a single bag; fm canned foods, to shampoo, to fresh milk,
to washing detergent, etc. Can eat cannot eat all in 1 large
bag.
If u wan to save e earth, bring ur own bags next time.