As Michael Kinsley sees it:
In a democracy, obvious lies and obvious liars should be self-defeating. Why aren’t they?
One reason is that the media have trouble calling a lie a lie, or asserting that one side is lying more than the other — even when that is objectively the case. They lean over backwards to give liars the benefit of the doubt, even when there is no doubt. Objectivity can’t be objectively measured. What can be is balance. So if the sins of both campaigns are reported as roughly equal, the media feel they are doing their job — even if this is objectively untrue.
But the bigger reason is that no one — not the media, not the campaign professionals, not the voters — cares enough about lying.