Because most men have a scarcity mentality when it comes to women. When a relationship ends for the average man, he has to start from zero. Back on the prowl, back in the courtship phase, trying to get the ass and hope something valuable comes out of his efforts.
Most women, even the moonwolves and fruitbats out chea, have stand by dicc 24/7/365. And if they don't, they have consistent offers on a daily basis to choose from. More importantly though, all women have friends who'll demonize the man for the demise of the relationship even if it wasn't his fault. So women have both emotional and physical reassurance almost instantly.
We men are alone, no one truly gives a shyt about us and most can't handle that fact. If we're not emotionally centered and happy being alone before we meet any woman, we'll usually do some ridiculous things when the validation we received from having a relationship is ripped from us abruptly.