You keep confidently asserting that, but many of the exact same compartmentalization and security procedures which protect the private keymatter also make a secret private key disclosure easier to keep hidden.

How would we know otherwise?

As you note, that "would be a more outrageous and damning discovery" - so there's more incentive to keep it closely held. It would help the NSA do what it feels it must, simply by using its other network taps. And, it would help minimize the risk of discovery without involving extra employees and ongoing connections, all while retaining the ability for Page/Drummond/etc to deny involvement.

Those add up to making a key compromise more attractive for NSA and Google than the alternatives.