Why doesn't the browser keep cookies organized by the website in the address bar? That way a cookie from a doubleclick resource received while browsing cnn.com is kept separate from a doubleclick cookie received while browsing imdb.com.

I guess I can answer my own question by saying it doesn't matter. If browsers did this, websites would be written so that an ad provider's resources are loaded with a querystring that identifies the user. So much for not being aggressively tracked.