Sadly I think it's sites like this and Twitter which focus on male nudity is contributing to more and more prosthetics being used nowadays. Let's be real, when a male does go full frontal, unless they're huge they're immediately called small. A lot of male confidence (particularly sexual) comes from the size of their penis. Also, look at how when male celebrities are interviewed when they do go full frontal how much attention is given to that scene. Remember these are actors, not porn stars or even models. They're there for their acting not for their bodies. Unfortunately not everyone has a big dick.
All that said, I'd rather they just not show anything that use a prosthetic myself, but I think those are some of the reasons why.