Regarding tech, I guess it's because it's still a field where (unfortunately) there are more men than women, and since men are attracted to women, advertisement

Regarding tech, I guess it's because it's still a field where (unfortunately) there are more men than women, and since men are attracted to women, advertisement agencies use this fact to capture the attention of the audience. It's these ad people that are sexist or at least using sexual attraction as a leverage. The day a field will have more women than men, they could switch to using semi-naked male imagery (even though there could be different dynamics in play that dictate the rules). The effect is unfortunate of course, because a male dominated field becomes less compelling for women and thus creating a negative spiral.