Well... people are stupid. Rather we are overly simplistic creatures that are set up to fail. My observations are:
A) nobody is able to discern whether or not something is true without an astronomical amount of effort and resources therefore we resort to deeply flawed heuristics
B) We are not rational creatures. Logic is mostly wasted on us. All decisions are ultimately emotional and often it's made by unconsciously substituting an easier question and deciding based off your answer to that.
C) Incentives are extremely powerful motivators. They can cause us to do bad, nasty things to get a reward.
D) A person sufficiently motivated by the incentives of being the sole person in control of all the resources can and will routinely use the flaws in our psychology against us to their advantage.
Humans basically don't stand a chance. The fact that England and the US happened is freaking miracle. Interestingly in all cases where they system of humanity converged on a democracy was when there was a broad coalition of people with different interests who would all benefit from opposing the ruler. The scary thing is that it can all go back the other way. Even the US can collapse and fall into this kind of nightmare. I think from 2000 to 2015 was the golden era of humanity and we are now on a slow and steady descent into madness.