The same reason planes remain airborne: beautiful, flawless design. We're built that way. Not to reduce men to a simple combination of parts, but we are nothing more than machines designed to inseminate you. We're part Don Juan, part caveman, all about OPP, and we came to party. And I mean filthy animal party.
Everything beyond that's just romance, and nature couldn't really give a rat's ass about that part. All it asks of us men is that we like our children enough not to murder them, and that we keep on spreadin' that seed, baby. Nature's kinda sex-obsessed. Like mother, like sons.
Of course, that's the basest possible take on man's natural drives. I believe it exists to SOME degree in every guy, and to an astounding degree in most guys under forty. Thankfully, society came along to keep us from acting on our every impulse. If it didn't, the world would be one big Hooter's-themed nudist camp/amusement park, and everything would be a lot stickier.
To a lot of guys, commitment means giving up on their secret (and remember, nature-imbued) goal of impregnating every woman on the face of the entire planet. Yes, it's a lofty dream, but the loftiest dreams can be the most painful to give up on.
Of course, commitment grants all sorts of new joys and freedoms, etc. etc., blah blah blah. But if you want to know why guys fear tying themselves down, it's not you, it's them. Them and their ambitious genitalia.
But before you go judging us too harshly, keep in mind that it's that same sexual drive that kept humanity going when there were only like six of us dicking around in a cave. Lord knows if we left procreation up to the ladies, our species would have succumbed to natural attrition eons ago.
NOTE: all of this is true only if you believe in evolution. If you're religious, then I guess men are just wild, hedonistic sin-junkies. Either way, pretty badass.