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.

Because it shows some thoughtfulness. 'I know you like x so here's money to spend on that'. Cash looks like you didn't bother.

tbf 95% of the time when I get a gift card these days it's Amazon or a big retail chain, that ain't exactly a deep cut in the gift department either.

We should probably normalize Chinese Red envelopes because honestly I'd take a nice envelope with a hand written note and some crisp bills over the annoying gift cards (https://en.wikipedia.org/wiki/Red_envelope)

Also some people struggle to spend money on themselves without guilt. Gift cards absolve that guilt as they can buy that thing without feeling bad about it