The issue is that only candidates from those parties are ever focused on. If there's someone who has great leadership qualities and would potentially be a great president, if they aren't a Republican or Democrat, then they will probably never be heard of. If they are a Republican or Democrat, and they have any sort of centrist ideology, then they will be torn asunder by their own party and won't get the nomination.
You mean like how the democrats would never nominate a presidential candidate who's signature issue is a law imagined by the heritage foundation and championed by the republican party as recently as 2006? A man or woman who would bail out wealthy banks with no strings attached? Who would pass tax cuts that gave far larger benefits to families earning 5 times the median wage ($250,000 /year) then the median wage? Someone who would continue the wars in Iraq and Afganistan to completion when ever that may be? Someone who would expect long term cuts to government spending in order to pay for short term stimulus to bail out business? Someone who would pass more stimulus in the forms of tax cuts (when you combine the ARRA and the payroll tax cut stimulus) then in government spending? Because those qualities nearly universally apply to every member of the democratic 2008 field.
American politics in one hypothetical conversation:
"Centrists": Democrats and Republicans should accept centrist principles x, y and z
Republicans: not a chance!
Democrats: well, okay if it's really important we can do x, y and z
"Centrists": Gee, what's wrong with Democrats and Republicans that they wont reach out to centrists like me?
Democrats: Huh? I said yes! Hellooooo? Can you hear me?
"Centrists": Yeah, such a shame nobody cares about me
In other news, the Senate is considering a law that would pair a whopping 10 billion dollars in money spent on an infrastructure bank with a tax holiday for multinational corporations repatriating money. Because just because people are actively camping out in cities across america protesting give aways to corporations doesn't mean we should stop shoveling tax breaks on them.