Alternative parties are disregarded because two major parties is the only stable state for the way our votes are counted. Without runoff elections or other ways of allowing people to express more complicated opinions, third parties simply cannot get traction. Many of us have complex political views but end up voting for a major party because that's what the system pretty much requires.

Certainly there's a lot of binary thinking in the US, but I don't think we're unique this way. An inability to handle subtlety is endemic to humanity as a whole.