Personal experience and mumsnet constantly demonstrates this to me.
One partner (more often men) suddenly want out and for whatever reason and then assume they can move out and re start their lives as if the last few years didn't happen.
They then have more money to themselves, find it easier to work and socialise, more leisure time, no need for a big family car, can start nice new relationships and all the fun and enjoyment that comes with dating pre kids. I could go on.
On too of this they still have the love and respect of their friends and family whereas if a woman does the same she is often vilified.
I think that joint parenting should become much more the norm rather than one parent left to do the majority of it.
Sorry for slight rant I just read one too many threads over the last few days.