The prenup can't help with alimony, but it can't do much about child support. No contract can supersede the law. Plus, if you were both working, she is unlikely to get alimony anyway.
As for the 'why'.... I think some women just believe that's the only way to stick it to a guy, right or wrong. He hurts her, he moves on like it was nothing, she has to hurt him back.... what better way to hurt a person than in their pocket? Plus, it's easy, accessible, legal.... it's a convenient way to exact revenge. Like I said, if I were a guy, I'd be extremely careful. The law just doesn't lean in your favor.
Sad truth.