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?
If I have a question [???] and the answer is "true" or "false". The probability of it being "true" is 50%; Every question you can come up with, you can come up with a negative version. e.g. "Is your name gwright" vs "Is your name not gwright".
So given any question where you know absolutely nothing at all about it, the probability the answer is "Yes" must be 50%!
The correct answer is the correct answer 100% of the time. And the wrong answer is the wrong answer 100% of the time. If I gave you a question where the correct answer is "no" and you knew _absolutely nothing_ about it, your answer "Yes" would be correct 0% of the time.
That is true, the probability of "Yes" is 0, but only according to you. Relative to me, however, the probability is still 50% chance.
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."
Down votes? I know physicist, who tell me in private, that they are afraid about a point of no return. That we heat the climate up to a point where the system gets unstable. This is, as far as I know, what could have happend on planet Mars and it could happen here. Yet, you barley see anyone mentioning this public.
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.
