Fake news would not be a problem if people knew it when they saw it. It matters because most people have trouble identifying fake news.
A Pew Research study (link below) showed that 64% of Americans had a great deal of confusion with an additional 24% having some confusion. That's a lot!
Fake news can have real life consquences.
In fact, the chart below demonstrates that fake news can get more traction than real news. During the 2016 Presidential election, fake news stories on Facebook were viewed 1.4 million times more than real news stories.
(click the chart to see it full size)