It's a combination of things, and it's difficult to think of / list all of them. People who never learned how to think critically (a failing of our underfunded school system), many of whom are aggressively anti-intellectual. People who feel like they have little control or understanding of the enormity of the modern world and grab onto simple (but tragically wrong) answers for comfort. People who think an evening of random googling makes them just as knowledgeable in various medical subjects as someone who went through many years of medical school (plus it feels good to spend an evening reading junk websites and feel like you know something those stupid doctors don't). People who get their "news" from random things they read on facebook instead of from accredited journalists. People who have been told for many years, by the leaders they've chosen to follow, "don't believe anyone else but me!" (you've heard the phrase "fake news" used - often to mean "news that's true, but inconvenient or embarrassing for the person calling it fake, so don't listen") - when said leaders then tell them, say, "don't get vaccinated! Bill Gates put microchips in it!", they immediately believe this wholeheartedly rather than putting any critical thinking into it. There's also a very strong tribal mentality, "everything my side says is automatically right and good, everything your side says is automatically wrong and bad" - so if one of theirs says something mean (and blatantly incorrect) to the nation's most knowledgeable epidemiologist, then, "hurrah for our team."
As an example, one conspiracy theory going around lately has been that "President Biden, on such and such a date, is going to round up anyone who isn't vaccinated and put them in camps." Well, if he had either the authority or desire to do this, don't you think it'd be a whole lot more useful, and less work, to just round them up, forcibly vaccinate them, and then let them go? But being put in camps sounds more scary - it plays into their fears more - so that's what the people starting the rumors go with. There is political power, and money, to be reaped, by keeping people fearful. And there is a certain segment of the population who are psychologically predisposed to be fearful - it's almost as if they are more comfortable when they're afraid and/or being vigilant about something (this also explains some of our country's over-the-top fascination with guns). And there are, unfortunately, some individuals who have a large audience and no morals, who are all too willing to use those people's fears for their benefit, either financially or politically.