It's not fake news - that's a boogeyman Trump invented to criticize news that doesn't flatter him.
Fox puts it pretty plainly.
https://www.foxnews.com/politics/what-t ... ions-againSo why did the public polls apparently once again underestimate Trump’s support?
“Trump voters are hostile to pollsters because they think the pollsters want to make them look bad. They wear it as a badge of honor that they refuse to cooperate and refuse to be interviewed. As a result, measuring the breadth of support for Donald Trump is tricky,” Luntz highlighted. “But to get it wrong twice in a row by huge margins is unforgivable.”
Veteran GOP pollster Neil Newhouse told Fox News that “there are a bunch of folks out there who don’t trust polls and therefore opt out of polls. They don’t want to disclose to others who they support.
“They believe that their responses will somehow be used against them or will be recorded someplace and attached to their name and specifically may not be comfortable telling someone else they are voting for Trump,” Newhouse, who served as 2012 GOP presidential nominee Mitt Romney’s pollster, highlighted.
The polls are wrong b/c they're getting garbage in. Garbage in, garbage out.