Because they can — and they know they won’t get called on it.

As Jon pointed out yesterday, Mitt Romney’s campaign has turned lying into something of a craft, if not an art form. But Mother Jones’ Kevin Drum offers an explanation why every time a politician tells a lie, he or she is likely to get away with it:

Politicians have increasingly discovered over the past couple of decades that even on a national stage you can lie pretty blatantly and pay no price since the mainstream media, trapped in its culture of objectivity, won’t really call you on it, limiting themselves to fact checking pieces like Kessler’s buried on an inside page. And because virtually nobody except political junkies ever sees this stuff, it doesn’t hurt their campaigns at all.

This discovery — that you can tell almost any lie without paying a price — is, in some sense, an example of national politics becoming a lot more like local politics. Blatant lying has always been routine in local races that don’t get a lot of press coverage, but the brighter media spotlight kept at least a bit of a lid on it in higher-profile races. However, with the splintering of the mainstream national media in recent years and the rise of the web and social media, national politics is local again. And being called on your lies by the occasional earnest fact checker now matters about as much as it does when a local columnist for a weekly newspaper calls you on it.

It takes a while for people to realize that norms have changed and to take advantage of it. Lots of politicians are probably still reluctant to lie too brazenly because they’re still working under the old rules, where the national media might call you on it and it might actually make a difference. The smart ones have figured out that this isn’t how it works anymore. Romney’s one of the smart ones.