As I was attempting to express on the radio, there's a certain imagined view of what the political center is which fits will into what the basic Beltway press view of what the political center is - somewhat economically conservative (or Liberal in the European tradition), and somewhat socially liberal. Miraculously this political center matches up just perfectly with the set of people who imagine a new wonderful politics above the partisan fray, and their agenda is presumed to be universally popular.
While this party of The Economist magazine (or, The Economist magazine circa 1997 when it wasn't as crappy as it is now) perhaps has its adherents, it's more likely that a successful third party movement (meaning, popular), would be more of an economic populist-nativist-socially conservative one. Some charismatic individual with a hybrid of the views of Ross Perot and Pat Buchanan who, coincidentally, were the standard bearers of the Reform Party...
It's a conceit of independence, of reasonableness, of untaintedness. It's very silly, and it causes people to rally around blank slate candidates who they imagine are also reasonable, independent, and untainted. It's why people vote for Arnold and Jesse Ventura.