Why can't CNN and Fox News stop talking about each other?

The Washington Post

The two cable news networks, says Paul Farhi, "are long and bitter rivals, of course, and have tweaked each other off and on since Fox News’s inception in 1996. But the crossfire has taken on new intensity in the Trump era. Hosts at CNN and Fox now trade blows almost daily about whose coverage or commentary about President Trump is more distorted or unfair."