It's the first thing we tend to do to all of the things around us: Name it.
It's true. And when you think about how much restraint a person would have to have to never speak a word during all that time spent together...he might have found it best not to name her, so he wouldn't be tempted to call her something.
Mind you, they may have had some of non-verbal "names" of a sort for each other, kind of like sign language. But my impression is that Cass would have never heard a single word spoken in all the time she was being raised by Cain until she ran away.