It’s because in the majority of cultures (and hegemonic cultures today) have had men at the top of their hierarchical structure and there are more straight men. The male gaze makes women into sexual objects, culturally we see women as inherently sexy as a result.

This is the same when you see little kids given dolls with different skin colors and told to pick the “good” and “bad” doll. Even children of color pick the white doll for the good one. Because “normal” or “default” means good. In the case of sexual attraction in a male dominated