Why German leaders believed Enigma remained secure

Why was the German high command so confident that its code machines, including Enigma, were unbreakable?

He attributes the confidence to faith in German engineering, reinforced by British-controlled agents reporting that the system was secure and by Allied restraint in acting on decrypted information.

Institutional ego is the starting point: German leaders treated the machine as a near-perfect feat of engineering and therefore assumed it could not be broken. That belief was reinforced because Britain had taken control of the German agent network in the United Kingdom and could feed reassuring reports back to Germany. The Allies also avoided acting on every useful decrypt, preferring alternative explanations or allowing events to unfold when disclosure risk outweighed the immediate benefit. Communications security depended on organizational belief and adversary deception as much as mathematics or machinery.