You seem to think that the 'spam problem' is technological. It's not. You remember getting junk mail in your snail-mail box, right? Same concept. There is a medium through which many potential customers can be reached, and is cheaper than the alternative (for paper mail, it's cheaper than going door-to-door, for e-mail, it's cheaper than paper mail).
Even if sender and receiver are authenticated properly, so what? A spammer will still be able to 1)forge his own authentication or 2)compromise an authentic box and use that as a zombie spam machine.
The only even faintly possible way to stop all spam would be to have all email pass through a single point, where spam could be stopped. However, that is nearly impossible considering the already widespread and deeply entrenched SMTP, and the fact that getting net users to agree to let a single company read every single email they ever send to anyone ever will be nigh impossible.
The spam problem is human. There is money to be made in spam, and in email spam, the profit margin is fucking massive. To kill spam, you must remove the monetary benefit, but the profit margin is so large, you don't really have much hope trying to cut that down to where spam doesn't pay.