Because she is a dog, and dogs do that no matter how disgusting we think it is. There are many schools of thought on this, but nothing has ever been decided on this matter. There are products on the market to help stop the dog from doing this and sometimes they work and sometimes they don't. One is called For-Bid, and another is Deter. They are powder or pills that you give the dog that are supposed to make their stool inappetizing to them (I know, you wouldn't think it would be to begin with, would you?). Most pet stores or vets will carry these products. Also try to keep the yard picked up so there are less of them for her to get.
I have been known to run across the yard with a pooper scooper screaming "MINE, MINE!!!" to my dog when he was about to eat one. Not something I'm proud of, but that is life with dogs.
