Because they're so simplistic and impractical. Consider, for example, the right-winger that wants to restrict Education spending, but substantially increasing the Government's powers to arrest illegal immigrants. Is that Big Government or Small Government? Or what about the left-winger that wants the Government to regulate business more strictly, but keep their noses out when it comes to police surveillance. Big or Small? Or the feminist that supports affirmative action to force equality, but doesn't want the Government to tell her she can't marry her girlfriend, or isn't allowed to have an abortion.
It's terms like that which make American politics look like a pantomime.
