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?
probability = ??????
probability = 0.50Of course stopping there seems to be silly since I'm sure more informed people have evidence that can shift that probability one way or the other.
So given any question where you know absolutely nothing at all about it, the probability the answer is "Yes" must be 50%!
It's just like if there was a stock the market know nothing about, but you know it just scored a magnificent deal a couple of hours ago, the market will see it as "meh, 50 50" (no price change), while you see "100% going to go up, must buy."
That's called insider trading, however. ;)
A sane risk assessment number should be something like:
Gain / (Risk * Damage)
So if the, theoretical, damage on Earth could make it something like Mars, we should act, even if we loose a lot of gain, that is comfort, and though the risk might seem low. On a sight note, the same goes for other humanity threatening dangers, like big asteroids.