Because it's a day to call attention to sexism against women. And there needs to be such a day, because the vast majority of the time, when something sexist happens, it's against women.<br />
Think about women around the glob. Think about your own society, even! Without even knowing what country you live in, I am 100% confidant there is more sexism against women than men. Probably WAY more sexism against women than men.<br />
And frankly, if you look at it from a perspective of what the day was intended to be (a day to call attention to the issue of sexism against women) and you decide it really isn't that necessary, I can only assume you are either particularly unintelligent or you really haven't ever critically analyzed the world in which you live.
We (men) get a whole month for International Mastur*atio* Month. Just don't forget to celebrate it.
because all the other days are mens day