It's not about people not knowing what's good for them, exactly, but rather with them not having the necessary skills or knowledge. The idea of them being able to make an informed choice is absurd, a lot of the time. Yet it's what they are asked to do.
Of course, some people have additional problems--they might, for example, be unable to manage their own households and so find themselves chronically in debt, yet they are asked to vote in what would essentially require a university-level course in global finance.