John Ehrenreich has a really good article up on Slate about why people believe in conspiracy theories and the real-world harm these beliefs cause:
Conspiracy theories arise in the context of fear, anxiety, mistrust, uncertainty, and feelings of powerlessness. For many Americans, recent years have provided many sources for these feelings. There's been employment insecurity, stagnating wages, and thwarted social mobility. For some, technological leaps and social progress-expanding views of sexuality, and racial unrest-can feel destabilizing. Then 2020 brought a once-in-a-lifetime pandemic, deep economic recession, widespread street protests, and a bitterly contested election. Any of these, taken alone, is enough to trigger anxiety, mistrust, and uncertainty. Americans are facing all of them simultaneously. For those who feel that everything is spinning out of control, a narrative that explains their feelings and encloses them within a safe community of believers comes as a soothing relief. Not everyone deals with uncertainty by turning to a conspiracy theory, of course. So, who does? It's not a matter of "ignorance" or "stupidity." People with low levels of education are more likely than more-educated people to believe in conspiracy theories, but such theories are also common among the well-educated. For example, about half of Americans with a high school diploma or less education say the theory that powerful people intentionally planned the coronavirus outbreak is "probably" or "definitely" true. But 38 percent of those with some college experience but no degree, 24 percent of those with a college education, and 15 percent of those with postgraduate degrees also support the theory.
There are a ton of reasons people believe in conspiracy theories, but lack of education does not seem to be a primary factor. “Why are people so stupid to believe this stuff?” is not a helpful question to ask, nor is it useful to assert that we just need to teach critical thinking skills.
Heart and guts win out over the brain all the time, and that is something that a lot of us on the left are going to have to deal with.