Because its about integrity. Openness is not about what you can get out of it its about showing your child that her connection to her birthfamily is important to YOU, even if for now, they cant understand that.
You wont teach your child that they have to be honest ONLY when other people are honest back or that they can lie, when others lie to them.
Put yourselves in your child's birthmother's shoes for a day or two and understand that the pain, the fear, the loss might just be too overwhelming right now and that YOU being consistent, full of integrity etc might be what she needs to heal enough to be someone your child can be proud of.
Openness is NOT about comfort and hopefully one day your children's birthparent realize this ...
I know how painful it is when contact isnt reciprocated. I was there for over two years. But I can tell you that the foundation I layed by being open, honest and consistent myself has paid off when it benefits my kids the most - NOW
Jen