cause beauty standards for women in Hollywood are different from those for men. and before someone says something about beefcakes and steroids, that's not for women, that's mostly for male power fantasies.
some dudes don't even realize how much thirst there is for characters like Phil Coulson, Bruce Banner (without the abs and all) and there there is Picard (aka Patrick Steward), the ultimate thirst trap. Chris Pratt was far more popular with women when he was a pudgy cheerful himbo, than when he beefed up and started acting more macho/playing more macho characters.
its not that women are never, EVER attracted to muscle, but rather standards of physical attractiveness for men/male characters are nowhere NEAR as ridiculous as those for women. even Magic Mike couldn't just coast on sexy dancing alone, they had to add some vulnerability and personality to their beefcake for more women to like them.
so yeah, the main reason these actors are beefing up is to present a power fantasy for dudes to project themselves onto, any sort of attractiveness to female audience is a secondary concern.
P.S. Hollywood creates expectations of beauty that are quite literally unrealistic naturaly. I won't claim all because I don't have acess to medical histories etc - but vast VAST majority of actresses and almost as many actors - had work done. multiple surgeries and all that jazz.