[Galadriel55]

The car was a bad example. I'll try another one. Imagine you're making a puzzle - actually making it, not just solving it. When you put in together, tho, you don't like the way a certain piece looks. To change it, however, you need to change the surrounding pieces as well. The result is that when the new puzzle is put together, it looks totally different without the bad piece.
When Sauron was destroyed, the last of the "Old World" had to leave ME, because the reoval of Sauron changed the world....and that makes the entire picture look different, not only that one piece of the puzzle......
Is this explanation better?

It is also possible that the ents' souls returned to Valinor. The souls - spirits? - of Elves go to Mandos, so why can't ents? They are living creatures just the same. That means that they'd have the perfect opportunity to reunite with the entwives. Maybe it's not specifically in Mandos, but somewhere in Aman. Perhaps Yavanna will make her own place to keep the souls of her creatures.