Most people in the US don't vote because those parties don't meet their needs.

Even if that's true that doesn't mean they all agree and would vote for the same candidate. In fact that seems very unlikely given how distributions usually look. And in a first past the post system people usually vote for the party that has the biggest chance of beating the party they dislike the most, not for the party they like the most, and over time that usually leaves the system with only two viable parties that a lot of people don't like. But it doesn't mean they all agree.