Reasons exist.... One reason is likely that they want to keep the valuation confidential outside of the company (people talk), because as a private company that information doesn't have to be public. It's not a great reason for the employees - but it's there, nonetheless.

It's tricky to share that with employees and not have it get out.

That being said, if it were all public, I think that would be fine, I'm not sure what results that would cause if every company did that, and it might be cool if every company were required to by law (hint, hint, congress!).