<troll>Wordy of a patent ? :)</troll>
Think people didn't care enough and where thinking inside the box ? Or maybe nobody wanted to do the hard work.
Really, all this is doing is updating the notion of a session cookie to account for the fact that tabbed browsing usage patterns mean that the browser process is usually much longer-lived than your visit to any one site. Browsers have long had the option to make cookies expire at the end of a session.