 Because it's abstract. It's far away from us. The problem seems so huge. We don't think we can do anything about it. We're not sure what to do. We don't quite understand how it works. Much of the time, people agree that the earth is big and beautiful and permanent. It has this permanency and strength and it cares for us. And that's not going to inspire altruistic aid. We have done studies where you separate people who tend to think of the earth as vulnerable versus don't know.