Because there is a huge chunk of feminists who perpetrate misandry in the open and are praised for it. Because TERFs exist. Modern feminism has done so much “reverse sexism” that some feminists avoid calling themselves as such in order not to be associated with those people.
Not to mention, if someone started talking about body positivity issues for men without mentioning women at all people would come swarming and say “women have it WAY worse!” like it’s happening in this thread. Is it only acceptable to talk about it if it’s prefaced with “modern society has a huge issue with female body standards, but”? And people would probably find a way to complain about that wording as well.