I would tend to trust /r/AskHistorians more than any other random people on the net about this. So here are some similar questions from there that were actually answered by flaired users:

What is it that has allowed anti-intellectualism to continue in the 20th and 21st century in the United States, when there's been such a strong push for everyone to receive a college education?

What are the roots of anti-intellectualism in the United States? What is its history?

p.s. As for my own answer, the documentary The Brainwashing of My Dad covers this subject quite well IMO, at least when it comes to the history of right-wing media and advocacy groups. And it's available on Prime Video (for free in the US). Just a heads up though; The trailer doesn't do it justice, since it makes the movie out to be incredibly sensational when it's actually rather serious, dry, and academic, full of sit-down interviews with people like Noam Chomsky, David Brock (Media Matters founder), Claire Conner (author of Wrapped in the Flag), etc.

It doesn't seem to be US-specific, nor is it limited to the last decade. Conspiracy theories and nonscientific beliefs are common in many countries. It might be that the US was somewhat free of them for a while, but I'm skeptical; it seems like wishful thinking. During the cold war there were conspiracy theories about communists, for example, and a lot of people were worried about fluorinated water.

One thing I've read recently is that in France, much of the population believes in homeopathic medicine. The government just stopped paying for it on January 1, and there is a lawsuit.

There seems to be a literature about conspiracy theories. Some papers find correlation between conspiracy theories and authoritarianism, which would make sense because authoritarian governments often engage in conspiracy.

And in a way, the popularity of conspiracy theories in black communities shouldn't be too surprising since people really are out to get them. Sometimes conspiracies are true.

It seems like people are more worried about conspiracy theories these days, though. They were always there, but it was easier to ignore them as a minority view.

One possibility is that people were more worried about religious beliefs before, things like cults and anti-scientific religious beliefs. It used to be a major subject of debate.

That's faded, so now conspiracy theories are more visible.

As others have said, it's not new and not unique to the US. There's a really good book by Michael Shermer called "Why People Believe Weird Things" that goes into depth about the topic in general. It explains the various logical traps that people fall into and why, and why it's also so hard to get some people out of them.

My personal thought is that the world has gone a bit bonkers, so while we were always susceptible to bonkers thoughts, it's now getting some level of intermittent reenforcement.

Suppose I went back to 1998 and said to a psychiatrist "Within the next decade the NSA will have setup a global computing network that captures all of the world's communication and a lot of every individual's movements to use for mass surveillance.". They'd put me on some pretty strong drugs thinking I was a schizophrenic.

Seeing that come to fruition makes you wonder just how deep those rabbit holes go. Learning more about the various three-letter agencies via declassified documents also makes you question just how far off some conspiracies are.

But also, as others mentioned, disinformation has been heavily leveraged for political means. It's always been that case, but it's easier to recognize as such now.

We're all dumbasses and I don't think anyone has a true scientific outlook on... well.. things in general.

"Anti-science" isn't a new thing. It's been around for so long. Probably, you've just matured and have been able to see these things. It is definitely not specific to the United States of America.

One could argue that it has always been part of the american culture/identity, at least since the first European settlers in the 1500s.

Kurt Andersen has written a book ( Fantasyland ) that thoroughly analyses the history of "rejecting reality" that is so prevalent in the USA, and to some extent in many other countries.

While the anti-science attitude was always there, it's true that it seems to be spreading much faster nowadays, and that is a real concern.

Famous quote by Isaac Asimov (1980):

There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that "my ignorance is just as good as your knowledge."

I've been reading Post-Truth by Lee McIntyre, who points to... oil and tobacco companies as a cause. Sowing doubt about whether there is scientific consensus on topics. Also conservative talk radio, and the general idea that non-experts can and should critique and question experts.