I would say because she was too old, and they needed kids for reproduction purposes? just a random thought. but a good point nonetheless. i think the point of the movie was that things happen for a reason.
AEthereal: yeah bro, cuz first me and my wife were pissed cuz we thought they were aliens too. lol but then we saw the wings, and then they dropped the kids off and they ran to the tree of life. so that relieved us lol