>A bigger argument though is that if they've had all these amazing successes why have they never told anyone? Why are all, or at least a lot, of their failures over the decades public but none of their successes?

Because it could expose people, secrets, or methods that are still relevant. The vast majority of former spies are still bound by various legislation about not exposing what they did - most of them have to have their books signed off by their former employers. It's why you should take everything a former spook says (especially people like John Kiriakou) with large grain of salt.

In extreme cases, it can also involve quasi-legal methods or cause diplomatic scandals. "All or a lot" of their failures aren't necessarily public, but only the ones that unravel in a way that gets leaked or exposed to the general public. I can guarantee you that the fuckups of every intelligence agency we've been exposed to is only the tip of the iceberg; even where there is exposure, we very rarely have the full story. Even for WW2 intelligence stuff, we're still getting drips of what "really" happened in many cases.