Because they've been raised by the establishment to think along narrow patriarchal lines. It's the ultimate proof that a bloke wants them. Besides, it's in their genetics, back to when the women needed men to protect them and their children from wild animals and warring tribes and famine. It's all about security and stability.
That and weddings are bloody brilliant. You get to get dressed up, get drunk in front of your family, eat good food, smash a piece of cake in your lady love's face, and dance until your feet hurt. Not to mention the bachelor party.