I don't understand it
Yes, you do. Bodybuilding attracts and is supported by a lot of insecure people. Insecure people have a long history of lashing out at what is around them because what they find in the mirror is too painful to deal with.
Women are easy targets; bodybuilding women even more so. A woman who has more discipline, dedication, and muscular development than the puny
men boys who complain about them are ripe, easy targets.