Fake news goes viral for a number of reasons, not the least of which are confirmation bias and also the fact that the vast majority of people who share news stories don't read anything past the headline.
There's also a strong contingent of people, whom we both know, who are utterly incapable of critically evaluating the credibility of a source. They don't understand the difference between substantiated and verifiable facts and arguments and conspiracy theory nonsense that has zero credibility and no substantiation. Your counterpoint at one time was, "some of that stuff has turned out to be true," but the fact of the matter is even a broken clock is right twice a day. If you throw enough crap at the side of a barn some of it is bound to stick. What should be clear is that we evaluate credibility so we can have a reliable look at what's going on. Wild-ass hypotheses stated as fact are completely unreliable.
So you combine click-bait headlines, confirmation bias, and the desire for some people to believe they're smarter than everyone else because they think they've found some top secret information that the ever elusive "they" are trying to hiding from mainstream media--those things all combine for a giant clusterf**k of misinformation and propaganda being shared within echo chambers online.
Some people are so divorced from reality that they will deny factual evidence when you show it to them. They will completely ignore the patterns in the data. Some even get angry at having their propaganda-filled assumptions challenged. The Allegory of The Cave has never been more apt. One of Trump's people recently went so far as to say, "there's no such thing as facts." This exposes a critical misunderstanding of postmodernism and deconstructionist theories. When social scientists or humanities scholars say that there's no "truth," they're not saying there is no facts. They're saying that there are many perspectives on those facts and different meaning ascribed to them. There are many "truths" experienced by those facts. Trump's people saying there's no such thing as "facts" is the rallying cry of propagandists who don't want people evaluating the facts the Liar in Chief, who objectively is the least truthful politician ever seen in American politics. They are literally arguing that they can create their own reality and stunningly ignorant people buy into that wholesale.
This is what the attack on intelligence and academia gets you. Mind-numbingly stupid people, who buy into the notion that you get to pick and choose what is real and what isn't. And that's how you get fake news going viral.