They are investing it - in real estate. And the WT Society has, for over half a century, earned cash through investment in the stock/bond markets.
The real question is WHO has control of the cash, investments and properties. I really doubt that the GB members have that control - just examine their resumes. A background in full time pioneer service, circuit and bethel home work without any specialized training or degree does not make a person the best one to manage billions of dollars of cash and assets.
ginger