1. Ivermectin is a legitimate miracle antiparasitic
2. Early in the pandemic, some researchers showed you can neutralize COVID in vitro with ivermectin. The dose is more than enough to kill a human.
3. Trump announced he was taking hydroxychloroquine (antimalarial/antiparasitic) to prevent COVID infection during the pandemic
4. A group of crank doctors saw an angle and at a Senate hearing announced ivermectin as a wonder drug that prevents COVID infection entirely
5. Right-wing influencers like Joe Rogan took the ball and ran with it, infusing further anti-vaccine ideas into the movement
6. The sleeper class, who don't pay attention to medical news and get most of their understanding of the world from the influencers they follow, submersed in America's simmering broth of reactionary beliefs and distrust of authority, pick up the news that ivermectin prevents COVID during the Delta spike, when fears soared.
7. To a certain class of people, having their beliefs questioned is inherently hostile. Everyone telling them to not take ivermectin only makes them more stubborn about it. Ivermectin is also really easy to get a hold of because it's a common veterinary drug and pretty easy to find if you're rural.
8. Those people infect their social networks further with ivermectin bullshit, because people will trust their friends more than they trust authority.

And so, five years after we conclusively know ivermectin doesn't help with anything but parasites, you still find people who believe in its global efficacy, which has become greater and greater over time in reaction to more and more proof against it. Because the believers are stubborn reactionaries.