Third parties are kooks who want power and hate each other. They would never unite.
Not true at all. In case you didn't know, the leaders of the smaller parties (McKinney, Nader, Baldwin, Barr and Paul) met before the elections and agreed on the following:
* An early end to the Iraq war, and an end to threats of war against other countries including Iran and Russia.
* The safeguarding of privacy and civil liberties, including a call for the repeal of the Patriot Act, the Military Commissions Act, and FISA legislation
* No increase in the National Debt
* A "thorough investigation, evaluation and audit of the Federal Reserve System."
Most of them know they won't win, but it's good that a few different opinions are being heard.
But to answer the question:
1) There's no reason for them to unite because they don't run to win
2) They have some ideological differences, even if they agree on important issues. If they wanted to join a party/coalition with no clear ideological orientation, they would simply join one of the 2 big parties.