Because it doesn't mean anything anymore.
One doesn't have to commit in order to get the perceived benefits of commitment. If you find the right 'dance partner,' you don't have to commit to get sex regularly. You don't have to commit to get somebody to do for you, to cook for you, to be an emotional rock or whatever else comes with 'commitment.' It's not required. The facade of commitment -- which isn't needed really -- is enough.
I believe men in general are creatures with a means to an end approach. Whether it's life, p*ssy, loyalty, etc... they will generally do what is required -- and no more -- to get a desired result. If I don't have to actually commit to you in order to get the things I feel I need from you in that stage of life, why commit? As
@FireDwaneCasey likes to say, commitment is the male vagina. And if we don't have to give it up, if we don't have to open that heart up and basically pledge an unyielding loyalty and desire to care for and provide to a woman in order for her to do the things we want her to do? Then we most certainly will not.
Meanwhile, women are giving up wifey qualities for bum ass nikkas and regular folk that ain't trying to cuff them like that. The same goes for women. If they don't have to exhibit commitment traits (or give up the box/nurturing side) for somebody to do for them, etc... why should they? Men are out here being emotionally available and providing for chicks, doing main nikka shyt for women that ain't worth the privileges.
You used to have to demonstrate certain qualities and a level of loyalty/service/providing in order to get certain things. You used to have to at least portray yourself as about something to get something worthwhile. You don't now. Society's different. Relationships are not the wave like that, and those who actually do want to commit to somebody? Chances are, they aren't able to attract the women they want to commit to, or ignore those who want to reciprocate that to them because they don't see them on their level.
Besides, committing to somebody will not stop them from walking all over your loyalty/allegiance to whatever you're trying to build with them.
What benefit is there to committing when it means nothing, and it will not stop heauxs -- man or woman -- from heauxing?