Allow Fields from connected objects in Calculated Fields

I received the following error when trying to write a formula in a field. Error: The field '$Project.C_BillingType' is invalid. You can only select fields from the current object.

Why can't I access fields from a Work Item's Project (such as billing type) in a calculated field? I can access these from Work flows, but not from calculated fields. This does not make sense to me, and is making calculations very complicated.