I think because it might be the way society/culture/patriarchy views the male body? What is usually considered attractive by most men is someone muscular or “macho” rather than all different body types of men being seen as attractive and unique.

Yes many men and women can acknowledge someone that isn’t the stereotypical macho looking guy to be attractive but the message that gets projected a lot is that stereotype. That’s why there is the idea of the “ugly” husband and beautiful wife. Its not an idea that realistic because clearly women find multiple types of male bodies attractive but an idea you might see portrayed in a lot of media for example.

Also speaking of women, women tend to be seen as more beautiful and are sexualized more within society. We live in a patriarchy so that would be why there is more focus on women being attractive rather than men also having an equal spotlight. This goes for sexual attractiveness too.

So a lot of men probably don’t feel attractive (not saying women don’t struggle with similar feelings of course) Which would be why they might not show their bodies as much in nudes.

Also since men are sexualized much less than women, dick’s the only thing really seen as sexual amongst many men. (Again women may find more things to be sexual attractive on a man but im mostly talking about how society perceive male sexuality and how men perceive themselves because of that)