Common sense: Take the two claims, 'the media has a right-wing bias' and 'the media has a liberal bias'.

Now, if one of those is true, which would you expect the media to repeat constantly? The one that's true and criticizes it? Or the one that's false that gives it cover?

Now, which of those two claims have you heard over and over and over, to the point many people accept it as true having heard it over and over and over (the big lie technique of repetition)?

The claim the media has a liberal bias has been repeated endlessly for decades in the media.

Isn't it funny that such a liberal media just can't shut up about its liberal bias? Oh, wait, maybe that suggests the opposite. Next up: "Fair and balanced" doth protest too much.

There are two major media sources in the US: right-wing media (Fox, 90% of talk radio, Sinclair, Washington Examiner, right-wing web sites, etc.) and corporate media (90% of media owned by four or five companies).

In other words, the very fact that 'everyone knows' the media has a liberal bias, is evidence it doesn't in this case, since that message has come so much from the media.

A factual review of the content leads to the same conclusion - right-wing and corporate agendas dominate.

There is a liberal presence - that's almost entirely what could be called 'fringe'. Rachel Maddow is now a rare exception, but she is under hostile management that has been replacing liberals with right-wing people.