The problem is that politicians have a lot to gain from really believing the stupid things they have to say to gain and hold power.

To quote an old thread:

Every politician I've ever met has in fact been a completely sincere person who considers themselves to do what they do with the aim of good in the world. Even the ones that any outsider would say "haha, leave it out" to the notion. Every politician is completely sincere. I posit that this is a much more frightening notion than the comfort of a conspiracy theory.

Cf. Stephen Pinker historians who've studied Hitler tend to come away convinced he really believed he was a good guy.

To get the fancy explanation of why this is the case, see "Trivers' Theory of Self-Deception."