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.
Because paper money is inconvenient as hell. Have you seen the size of those 25ct coins! And then you go to buy the cheapest bottle of water, and you need 20 of them to buy half a liter.
Oh I see. But I mean why not just a debit/bank card that allows you to pay with money you have but not with money you don't? You just meant card in general. Yes I agree cash is now very inconvenient.
Basically, you're less protected against purchases gone wrong with a debit card or against fraud. A fair number of (mostly higher income) folks also get other benefits from credit cards--though that may not be relevant here.
However, a debit card usually covers the floor of needing a card of some sort to pay for a lot of things.
I suspect a lot of people in the US use credit card generally to mean card of some sort because credit cards are so common. You do mostly need a card of some sort (and a smartphone) for many purposes but it mostly doesn't need to be a literal credit card so long as you have enough money in the bank.
