That statement is predicated on the mis-guided assumption that politicians are a statistically-representative sample of Americans.

No one thinks their own frieds are corrupt. That's part of the corruption. The quiet benefeciaries "just following orders" and "useful idiots" of corruption aren't seen as corrupt.

Invalid logic. It is not necessary for most if not all people to be corrupt for the same to be true of politicians. Many people are gullible. Many people are easily manipulated. This is especially true of those who are young, and prone to adopt the ideology of the group. But yes, I do believe that a significant portion of the population possess the brain anomaly that manifests as narcissism and these people are more successful than others at climbing political hierarchies because they are unrestrained by conscience.

If you are curious about this phenomenon, you might want to research "the flying monkeys of the narcissist"