It's a combination of never having learned the basics of science and now seeing the falsehoods they've been fed as equivalent to science.
Take the Tylenol thing. You can explain to one of them the scientific method, what a survey of studies is, why correlation often appears when there is no causation, etc. I experienced this last week: at the end of my explanation, the person (a 45-year-old) replied that he "simply disagreed."
The coal, the climate, etc. are all the same. There is a broad sense that because they've been convinced of the value of expanded oil drilling through lines like "Drill baby drill," their current perspective on it is of the same merit as actual scientific research.
I think it's simpler than that. They believe in what they believe specifically because it contradicts the views of people they dislike. I guarantee you that the person you were talking to got a real kick out of you wasting so much of your time trying to explain a position.
However, these people do have a weakness. They feel good when they win the attention economy and the emotion economy, and those are actually really easy to subvert with a little out of the box thinking.
"I'm glad they've finally figured out the cause of autism."
"Chemtrails?"
"No, they said it was Tylenol."
"I don't think so. Did you know that the number of chemtrails the government has put into the air has increased 7-fold since January?"
My experience has been very different from the one you're describing.
The person I was talking to is someone who cares deeply for me (and whom I care for deeply too), someone I've known for almost my whole life. He wasn't having fun contradicting me. In fact, it was making him visibly uncomfortable to do so. He was engaging in the conversation in good faith. He just doesn't have the foundation to understand what he doesn't understand. I'm optimistic that even though he came away still disagreeing with me irrationally, there is a chance that by exposing him to a fuller explanation, he'll seek out more information for himself at some point in the future.
I suppose I am fortunate that I don't know anybody who is that far gone that cares deeply about me.
However, the people I use this trick on aren't strangers. They are regular acquaintances that have conspiratorial views, but think I am one of the "good" ones. When these people tell me things like it's just a difference of opinion, you can tell that they derive strength and satisfaction from their ignorance.
My goal isn't to convince them, it's to stop them from reaching into their bag of conspiracy theories when talking to me. In that, it has been wildly successful.
I can see that your approach would be effective at shutting down conversation and stopping people from telling you their wild conspiracy theories.
I think that with my experience, I've had to recognize how fragile some of the most important incentives are. Like the safety that underpins trust. To have trust, it needs to be safe for people to be wrong. That means I often have to listen respectfully to views that I find abhorrent, in order to get to the point that I can share my own thoughts fully.