well probably because it’s a lot easier for girls to pull it off than guys. a girl can add things to her body to make it more “male” but it’s a lot more difficult for a guy to take things away and make his body more “female”
see i actually think of it the other way around! a girl has to minimize aspects of her body - breasts, hips, face shape - while a guy has to “add on” in those areas. i’d say it’s probably around an equal difficulty level to go from either gender to the other, honestly.
i think probably what throws a lot of guys is that cosplay involves a lot of traditionally feminine skill sets - sewing, makeup, hair styling, etc. but cosplay is a lot like theatre in that it involves trying to become a character; i dont think it’s asking too much to say a guy could become familiar with these skills.