Humans were never good at critical thinking. Evolution built us with cognitive biases and heuristics, tendencies in behaviour and emotions that help us in our survival, but don't necessarily help us perceive reality as it is. Clear thinking was never the measuring stick. People with higher cognitive skills were likely selected because intelligence was useful for survival. Basically it's one phenotype among many, and it's a side-effect that we're able to engage in critical thinking. But there are many ways to play the game of evolution, e.g. social influence, hard work, athletic skills, physical appearance, leadership abilities, deceitful and aggressive behaviour, risk-taking, etc. There are always going to be a lot of dumb people because they passed through the sieve of evolution using means other than cognitive skills.