They aren't, debit cards are the default. Credit cards are just readily available and people like having credit.
Credit access in the US is ludicrous, you can be a terrible borrower (late or even tons of charge offs) and still get credit offers for credit cards. A lot of people then get into trouble because somewhere in their psychology the credit limit counts as money they "have" even though they understand they have to pay it back. That's why you have people talking about literally freezing their cards in a block of ice to control their spending.