It isn't the question of why, but how. Of course, that would be the ideal
solution now wouldn't it? A slight problem. As
novalis puts it so
eloquently, the
US electoral system is designed to be a
two party system. In addition, there just may be no
grass roots political organizations any more. Not in the
national stage or at the
state level anyways. If you want to
win, you go with the established
political organizations.
Let's expand a bit, into the rest of the world. In Israel, parliament is designed to have multiple parties, not the two party system. But in order to get any legislation passed, there must be a majority right? So one party must lead a coalition of many parties to do anything. I don't keep up with Israeli politics but the last party to hold majority was Mifleget Ha-avodah, the labor party. I could be absolutely wrong, again, I don't place high priority on Israeli politics. Other examples of this type of political organization can be seen in India. This makes it even more difficult than the US to do anything (and you think Congress was slow). Two party systems (e.g. England with a sometimes three party system, Australia, USA) are probably a lot better than what other democratic countries have.
Example 2: Australia. When the Greenies or the Gun Party wins is the day Melbourne becomes a better city than Sydney, Tasmania sink into the ocean, and New Zealand beats us in cricket. It just isn't going to happen. Two party system = Third party never ever ever ever win. Ever.
It sounds like a nice and easy solution, to kick the parties out, be it 2, 10, or 500 parties representing. But it isn't possible. First of all, because the large political organizations known as parties will always exist, due to the size of the whole system. And since the American system is designed for two parties, there will be two major parties to choose from. Idealism is dead in large scale politics anyways. Just go for practicality. It kind of sucks to go with the system, but hey, bar thermonuclear war, intervention of God, or some massive military coup, it isn't going to change, no matter how you try to fight it.
And besides, I was a wee lad of 12 years in Sydney Australia when I realized that both the Labor and Liberal parties are both full of shit. Paul Keating was semi-decent but did absolutely crap, and John Howard was and is still a lot worse. You're right, this parallels to the US system, both parties suck. But your solution of kicking both parties out is both way too idealist as well as impossible. Life is unfair. If you can't beat them, join them.
In an ideal world, I would be either in front of my computer, at the beach, playing paintball, and having sex, all the while doing copious amounts of soma and not give a care about anything. Screw politics. But idealism is dead. Sucks doesn't it?