It's annoying that they don't cover clothes with something. I've got stains on new clothes before reusing bags that have had food in.
Yes, I completely get the point with bags for food, as most people go shopping for food every few days/week/fortnight and need to buy a lot, and don't find it especially thrilling - and it doesn't generally matter if you spoil food packaging, and veg can be washed anyway.
The thing with clothes, though, is that (most) people buy them far more seldom than they buy food, so you don't have a regular 'routine' to it, like you do with your regular food shop.
Also, buying clothes is something that many people enjoy doing and spend a lot of time planning and finding the right outfit.
It can seem like a huge anti-climax to have spent ages finding the right, maybe delicate garments and then being expected to whip out an Asda bag that you last used for some sweaty carrots that were near the turn (now that they no longer put dates on them) and a bottle of ketchup with some smeared on the outside from another bottle that exploded in the packing case in the delivery lorry.
Depending on the garments, supermarket bags are also not usually big enough to safely accommodate a coat or a dress, without you having to stuff them right in any old how.