Because everybody from Decathlon to Body Shop wants to make it easy for you. Muji’s got a slick little nylon shopper that fits into its own (slick little) pouch for 8€ or a bigger sturdier bag that fits into its own clutch for 30€. Body Shop had great canvas bags for just 2.50€ and Decathlon has all sorts of options (the best is the nylon bag for 1.50€ although the hard plastic ball it folds into is a little suspect.) BoPreu supermarkets will discount the price of the normal plastic bags from your purchases if you bring your own.
And of course Ikea, ever ingenious, lets you buy a version of their big plastic bags and even has cute designs for schlepping and totting for .99€. (Calm down, nobody’s forgetting about the traditional options either – the wheelie cart and the straw basket are still as good as ever.) Just remember next time you’re shopping, in Barcelona or anywhere else, to say no to plastic bags!