I used to believe that too. You might call it the unwieldy conspiracy principle. But I'm not as certain of it anymore. Things like the Snowden revelations might seem evidence for the principle but I kind of see it the opposite. For years, thousands even tens of thousands of people knew about that and they still kept it secret. It's entirely possible it could've gone on for years more without a particularly conscientious person happening to be let in on it.

The same goes for the Pentagon papers. And COINTELPRO was only uncovered due to people actively breaking into an office and stealing records.

Who's to say if stories like those are inevitable conclusions that befell all the major conspiracies worth writing about in our recent history or if they are just some portion of ones uncovered. Similar maleficence has persisted in the private sector too.

I'm not saying for sure they are listening in but at the same time, I'm not sure I buy unwieldiness as a surefire principle demonstrating it is not happening either.