They hear about one pop star's cousin [1] in another country that had swollen testicles after his shot and that's someone that not only they can relate to, but even trust more that official sources because, hey, why would that star lie about it. While many are convinced that the government and health care industry in general has an ulterior motive.
It's a combination of disinformation and lack of critical thinking skills. I spent 2 hours on the phone with my brother one night, pointing out why his "sources" were not really authoritative sources - if you can't find the source of their data, don't trust it, "I know a guy who..." is not a "source". Not sure if that's what swayed him, but he got his first vaccination 2 weeks later. And it's a good thing he got it, he and his wife just recovered from COVID which they think their daughter brought home.
[1] https://www.huffpost.com/entry/anthony-fauci-nicki-minaj-cov...