David Corn, the Washington editor of Mother Jones magazine, was asked this question last week while doing a Reddit AMA (Ask Me Anything) session:

Dirty Projector: Why don’t more journalists call out politicians on their bullshit? Most politicians flat out lie to the public, and no one holds them accountable. Is it a fear that they will be black listed?
permalink

David Corn: Good question. This is something I’ve pondered and written about many a time.

See
http://www.motherjones.com/politics/2012/09/factcheck-politifact-lying-politicians
and
http://www.motherjones.com/politics/2008/09/after-w-will-press-get-spine
I think there’s a lot of factors. Beat reporters often don’t like to be too antagonistic toward their subjects, for this will affect their ability to get information from them. MSMers often feel it is not their place to render such judgements and believe it is their job to convey what the pols are saying and let the public reach its own conclusions. I disagree (what a surprise!) and believe the press needs to be aggressive in policing the pols and in calling out untruths. I’m heartened to see more of that in the MSM, via factchecking (see the first piece above), but this recent trend in the media has had its rocky moments.