Same reason they gift you a book instead of a can of petrol. By giving you a gift card, they're forcing you to buy something sold at a specific store chain, not to buy more petrol.

It can also be a way to make sure e.g. “fun money” gifts are actually spent as intended, getting around things like sense of responsibility, overbearing spouses, etc making the recipient feel obligated or pressured to spend it some other way.