Because it is easier to be on the wrong side and have people forget (again, and again, and again) then to have to admit they were wrong in the first place.
That is the only consistent thing that comes from the press. The belief that nobody else ever admits to their mistakes...and that they themselves never actually make any mistakes.
I really, really dislike the press. Not the concept of the press mind you, but the way it has been done for the last 40 years or so.
Now the media don't even have the pretense of honesty. They lie with impunity, and nobody stops them.
But the good thing is, that most people don't believe them any more either. They are imploding, and are too stupid to notice, or care.