What makes you confident that is the reason? Are all people equipped to work out what is obviously false?

It is one thing to be distrustful of official sources and another to believe that Bill Gates is injecting nano particles into vegetables based on a video you saw on YouTube.

People don't just start believing Bill Gates is injecting nano particles though. They get there through a gradual process.

Edit: And I am confident this is the reason because I have observed people go down this process. People who are intelligent, and not the caricatures they are made out to be.

Could you describe the overall process you have seen people go down?

You're right - the Bill Gates injecting nano-particles into COVID vaccine has its basis in several pre-existing conspiracy theories, such as anti-vaxx, fears of RFID/"Mark of the Beast" tagging, Satanic/ritualistic activities by elites (see: Bohemian Grove, Skull and Bones, etc.) which themselves are derived from (albeit not always directly indicative of) anti-Semitic conspiracies.

Even with QAnon, the pieces were already there among the right-wing and extremist Christianity, they were just assembled into a single, somewhat coherent narrative.

This. When information is suppressed, people fill in the gaps. Some people do it with wilder and wilder ideas, but there _is_ a gap, and those who think the information suppression is totally benign are naive, considering both historical precedent and the obvious question of, "what are they hiding that they don't want people to know about?" If it was benign, a government by the people and for the people would be transparent to the people. Lack of transparency hints that it is neither by nor for the benefit of the people.

No conspiracy theories thrive when the truth nuanced and complex, and takes more than two sentences to explain.

For example, rare and unforeseeable side effects from vaccines. They exist. Information is not being suppressed. However, the risk-benefit calculation from both a personal and societal standpoint is just complex and muddy enough to be exploited by some snake oil salesman hawking nutritional supplements that boost “natural immunity”.