Because Attack of the 50 Foot Woman turned giant women into a symbol of feminism, making it something palpable to the general public, while giants… aren’t.
It gives giantess as a concept a foothold in the public imagination to start from, and then just showing skin is enough to get a lot of guys into it because, well, boobs.
Meanwhile, the equivalent is Incredible Shrinking Man, so the concept of a giant man doesn’t have much to go on beyond ‘big movie monster’, and honestly the giant man adjacent concepts have basically been absorbed into King Kong, since the pre-Monsterverse versions have him grabbing and prizing a tiny woman in all the ways you’d expect.