Who's saying we don't? There are plenty of one-dimensional, stereotyped male characters and those used as just another muscle-bound hunk, and I would cheerfully point out where those are stupid too. But this issue certain disproportionately effects women, as a women's sex is regularly treated as relevant to any and all things she does (see here); a woman's objectification is more indicative of our culture's treatment of women in general. Men are indisputably privileged when it comes to sexual discrimination. While there are objectified men in various entertainment mediums, women are disproportionately affected by sexism in real life.

I'm not saying that men are never objectified; I'm just saying it's not as significant, and I'm not putting forth the effort to get up on my soapbox about it. Nor do I do so on the vast majority of other issues I have an opinion about.

But on the point of how privileged men are: what would you think of a white person who went into a thread criticizing certain forms of discrimination against black people in the US and said "white people are sometimes discriminated against too! Why aren't you protesting about that?" Why isn't that question treated just as silly and offensive in a discussion about sexist portrayals of women?