How can the left say there's not a leftward bias in the mainstream media?

See, the left doesn't think it's biased, which is part of why they're so dangerous. Ronald Reagan said something to the effect of the trouble with liberals isn't that they're liars, it's that they know so much that just isn't so. No matter what the topic, a truly rabid liberal will figure that what he believes is absolute truth and anything other than that comes from a biased source and is therefore untrustworthy.