They have very strong institutional motives to be seen as highly-competent. A cynical view would be to think of intelligence agencies as semi-rational actors at

They have very strong institutional motives to be seen as highly-competent. A cynical view would be to think of intelligence agencies as semi-rational actors attempting to capture as much of the budget as possible with secrecy reducing all the costs of getting caught lying (think FEMA with no transparency and machine that prints get out of jail free cards). The cynical view is never fully accurate, sometimes it isn't accurate at all because organisations always include idealist/true believers/patriots. Organisations which don't include idealists tend to weed themselves out pretty quickly and collapse.