Politicians are not sufficiently educated to know they are being stupid
Crypto needs math literacy to understand. SERIOUS math. Not high/grade school, but University Major type math.
Without that background, (assumption - probably safe) politicians have to rely on "experts" to advise them, and they get to not only pick the experts who may not have the required math (assumption - reasonably safe), but the politicians will keep asking until they find an expert who supports what they want to hear (assumption - proven).
So there's no way to tell them it's impossible that they will listen to - they think those that are telling them "Not possible" are either i) hiding something, ii) have vested interests, iii) are being paid by the opposition, iv) are terrorists and shouldn't be listened to anyway as that's who they want to spy on...