Earlier quoted context omitted.
It is probably because people are comfortable with the familiar. Sure, a warmer earth might be a positive-sum change, but there's an equally good chance that it won't be. So why take the risk of losing what you have if you can avoid it?
> equally good chance that it won't be. probability = ?????? is not at all the same thing as probability = 0.50
So given any question where you know absolutely nothing at all about it, the probability the answer is "Yes" must be 50%!