Who do bros who lift get the reputation of being stupid? Do people think all the blood from your brain goes to your muscles if you work them too hard? Maybe it's just that gym bros tend to be pretty laidback and happy, and therefore a little simple. In the words of Elle Woods, "Exercise gives you endorphins. Endorphins make you happy. Happy people just don't shoot their husbands. They just don't." There's a lot of truth to this. When you're consistently working out, you don't have a lot of room for dwelling on negative emotions, because you get them all out when you're exercising. That's why people yell and grunt during exercise, it's all the bad vibes leaving their bodies.
Of course, it's still possible to be a miserable meathead, but you just don't see them that often. It's also because gym bros know they need their daily dose of memes to keep them sane.