The narrative comes from the medias inaccurate and misleading coverage of the polls in 2016. Many news outlets all but declared Clinton president before the election.
But the media is not Nate Silver. He said Trump had about as much chance of winning as the Cubs had that year of wining the World Series, and obviously both happened.
Silver did a nice writeup of the whole experience: https://fivethirtyeight.com/features/the-real-story-of-2016/