Fanaticism and general insanity is in the very definition of the country, though most people can't see it anymore. The Puritans who came to the Massachusetts Bay Colony weren't seeking "religious freedom" unless you count the absolute autocracy of one religion as 'freedom'. They were totalitarians who couldn't stand living in societies that had differing views from their own. They left England because they couldn't force their way on the society there, and lived in Holland for a while but couldn't stand actual tolerance. So they were trying to found an exclusive 'pure' (as in Puritan) society of their own. There is a reason that 'Puritan' has the connotation it does today. And the strictures in the Constitution about the President having to be native-born were based on paranoid fears of some foreign power slipping a ringer into the US political system and taking over the government. (And in the end, the foreign power co-opted a native-born American anyway! Hah!)
There were very wide-spread dueling conspiracy theories in the run-up to the Civil War. In the South "everyone knew" that there were Abolitionists sneaking around fomenting slave rebellions. Because of course slaves would not rebel on their own, they weren't smart enough. John Brown's raid on Harper's Ferry (October 16-18, 1859) was perfect fodder for the paranoia of Southerners. Southern ideology required that all troubles come from outside. Meanwhile, Northerners were in fear of the Slave Power, the monolithic conspiracy by the Southern elites to spread slavery everywhere. The Dredd Scott decision (March 6, 1857) struck down the legal restrictions preventing slaves from being taken into Northern states, setting off Northern paranoia that had already been growing for decades. And the rest, as they say, is history...
After the Civil War the paranoia continues. The very real Ku Klux Klan promoted all sorts of conspiracy theories as part of its campaign to end Federal control of the conquered states, and the growing industrialization of the US leads to both anti-labor and anti-capitalist conspiracy theories. With the outbreak of the Russian revolution, "Red Scares" come into vogue.
More recent history you are probably familiar with. Cold War paranoia had Communists everywhere. World-wide conspiracy theories always find a good home here. Paranoias that originated in Europe, like Antisemitism, Antimasonry (Hello, Milo, where-ever you are), and more get plenty of believers in the land of the free and home of the brave. This bulletin board is like cultural flypaper, attracting every kind of nutty buzz out there in the current popular culture. You are familiar with a wide range of the nuttiness out there just by being a regular here.
The outlier that makes you and many other people think that "America" is somehow a bastion of sanity and civilization is the period that founded the modern era, Franklin Roosevelt and Eisenhower, and to a degree Harry Truman, did an amazing job of coping with the crises of the Great Depression, World War II, and the Cold War confrontation. But even their administrations were struggling against the nutty movements inherent in American society. Roosevelt was called a Socialist and a Communist by the far right, of course. Eisenhower, the president who signed legislation making it a crime for Communist to run for public office (eventually struck down by the courts), was called a Communist by Robert Welch, the founder of the John Birch Society. The nuttiness has always been strong even at the height of American power. It's not that this has increased, it's that the ability of a small core of rational people at the top to control it has been eroding.