Fox viewers prefer to be reassured that their ignorance of reality is correct. They'd rather not know the truth about basic facts if they would then have to surrender any of their bullshit opinions.
By taking sides so brazenly, Fox has gained audience share at the expense of turning itself into a big fat political target. The establishment political press is far too timid and clubby to have made this discovery on its own.
But if the White House says something, they have to cover it.