It's because people are now getting much of their news from social media (often exclusively from social media), and are NOT clicking through to the original source. So they see something that someone in their network reposts, and it is indistinguishable on its face from "real" news, and they believe it.
A lot of people, for instance, believed that story that was going around that the Pope endorsed Trump shortly before the election, or that story that an FBI agent who was investigating Clinton had been found dead along with his wife in a very suspicious apparent murder/suicide.
The site that the Pope story came from said right on its "About" page that it "[...] is a fantasy news website", and "Most articles on wtoe5news.com are satire or pure fantasy". That part doesn't get tweeted or facebooked, though, so quickly gets lost.
The source for the FBI agent murder/suicide was a newspaper that doesn't even exist. All it had was a fairly new website with a handful of fake stories and a bunch of broken links.
It used to be that "fake news" didn't spread far. If the National Enquirer proclaimed that the government was using semen from a captured Bigfoot to artificially inseminate human women in order to try to breed super soldiers, for instance, it would amuse people who say the paper in the checkout line at the supermarket, and a few readers would believe it, and maybe tell their friends in person, but it wouldn't go much farther.
With social media I bet you could get a large number of people to actually believe that there is a Bigfoot semen based super soldier program.
Getting news from social media is the equivalent of in the pre-internet days getting news from idle chat at parties and overheard conversations of the people around you. We all did get news that way back then...but we also subscribed to newspapers and news magazines, and listened to radio and TV news, and would match up what we overheard with those sources, and for the important things could figure out what was likely true.