Going back on your word

Lately I've been realizing how very blessed I am to have placed my baby in an open adoption that stayed open, as promised. I hear from so many birth moms who have placed under the impression that the adoption would be open, only to have the adoptive parents go back on their word. I just can't understand why that is. Why would you promise a certain amount of visits or updates and then not follow through? Unless it's for the safety of the adoptee, I don't see any reason that would make it okay to cut a birth mom out, especially without communicating with her about it first. Why does this happen so often?