Why is it that woman who are fat after having a child or children blame having a child as the reason?

Being a mom of a 9 year old who plays sports, I hear this all the time. I also see what they eat on a day to day basis and see the lack of exercise. It's funny bc out of 12 mom's on my child's team, I'm the only one running around with the kids. And guess what; I'm 1 of 2 thin one's. 9 of the remaining 10 or obese. Gee I wonder why!