I think the reluctance, thanks to that period, and the fact that most folks basically inherit their politics, is largely a kind of "cultural inertia", akin to turning a tanker. A society's political climate changes slowly (outside of crisis). But the change is happening. Much to the right's chagrin.
And then we have the confounding matter of fiscal vs. social liberalism/conservatism. I don't know how those on the Gallup poll shown were interviewed.
I think you are vastly overstating the belief that society as a whole views "liberalism" in a negative light, and therefore people are pretending they are centrist. The more likely view is that they, indeed, do believe that they are centrist and haven't really done the analysis to see where they fall on the spectrum.
As for the Gallup poll, it is likely based on political leaning in general, not fiscal or social issues - most people do not distinguish (though they should).