Because people use credit cards for the rewards (cash, mileage, whatever) or because they don’t actually have the money now. They don’t want to pay for a unique number for every transaction (which doesn’t actually preserve privacy since most of the stuff you’re buying online needs a shipping address) nor do they want the money immediately pulled from their bank account.
Credit cards give rewards which can be significant, reduce or eliminate the need to keep liquid cash around and frees it up for investments which compounds on top of the rewards, and it dramatically simplifies the number of annoying bills from N to one.