Why would Fox News report anything negative about GOP or CNN about Democrats? Media checks and balances are other orgs of different beliefs. Dangerous to rely on only 1 source or even 1 partisan community.
I agree that it's vital to consume heterogeneous media, but as for 'why would they': Because they should not be mere propaganda machines.
Incidentally, I have seen anti-GOP coverage on Fox and anti-DNC coverage on CNN. Not enough, but a non-zero amount. Maybe the motive is simply to give the false impression of impartiality, I don't know.
