Because people are reliant on debt because wages haven't kept pace with expected standards of living.

Of for more ago well off people some cards offer perks like cash back or air miles and it's free if you pay it off in full each month.

Also, in my country at least, you get more protection if you have an issue with the goods you've bought. https://www.moneysavingexpert.com/reclaim/section75-protect-your-purchases/