Again, because they are convinced that it isn't happening. If it isn't happening, as they think it's not, then there's no need to take ANY pre-emptive measures because they honestly think that it will NEVER happen, or at least, not in the conceivable future.
Now, I am not a climatologist or a meteorologist. However, I AM a biologist who works next to a building full of physicists, chemists and, yes, climatologists. I speak to these people, and the OVERWHELMING consensus among these folks is that it is actually occurring, although they are hesitant to say exactly what the effects will be.
Sceptics of Climate Change have their arguments, and I've heard them. And virtually every single one of them has been shot down, definitively. The Sceptics just keep on using the same arguments, again and again and again and again and again. It doesn't matter that their arguments have been proven wrong (I.e They keep claiming that Temperature rises are due to extraordinary solar activity, when in fact, the sun has been at its most dormant for decades) or make no sense (sometimes they say there IS no temperature rise, and then they proceed to then blame said temperature rise, which they previously said didn't exist, on the sun, which, again, has been proven FALSE). They're like the creationists in that respect - it doesn't matter how many times you shoot down their arguments, they cling to them. It doesn't matter how many hundreds, thousands or even tens of thousands of experts you get to prove they are wrong, they will always find someone with a degree (often in a field unrelated to the matter at hand) to say that it isn't happening. Creationists LOVE to do this: They get an engineer to say Evolution is wrong - they don't get a molecular biologist, or a geneticist, they just get anyone with "Dr." attached to their name to speak out against evolution, even when said professional knows barely anything about evolution. Climate deniers do exactly the same thing: they get mathematicians, geologists and physicists, who barely have ANY connection to climate research to speak out against it.
Climate deniers simply don't want to believe it is happening. Also, and this is the Greeny-people's fault, they are sceptical because many of the claims of the Greeny-folks have been proven wrong (i.e, especially the claim that we'd all die of starvation in the 60's) in the past.
You know what, Climate Deniers? Why don't you go to this site:
http://www.skepticalscience.com/
Why don't you ACTUALLY READ what the Scientists say, instead of just swallowing whole whatever the Climate Deniers say? Hm? Why don't you go do that?
There are a few smart people who say Global Warming isn't happening. There are a lot more smart people who say it is. Granted, occasionally, the lone voice is right. Most often? It's not.
Personally, I don't think there's a damn thing we can do about it. Well, actually, there's a lot we could do to stop global warming, but there's nothing politically or economically viable that we can do. So, we're screwed. The public are ignorant and are all too willing to swallow the arguments of the Skeptics, which amount to "DO NOTHING, PROCEED AS NORMAL", which is what people like to hear. So nothing will be done, politically, about Global Warming. The politicians are powerless in the face of the people crying out for not a single sacrifice to be made.
Once again, science will have to step up, as it always has to - saving the people from themselves, once again. Science has done so much for people, yet people always find a way to spit in its face - mostly because they don't trust us "Book-learny folks, what with their glasses and fancy words! Think they're done darn to good for us! What're they know! I dun need to know all I need on the farm! Them egg-heads don't know jack all! Think they's better thans me! I'll show em! I'm purposefully not gonna listen to em! That'll show those poindexters whose boss! ME"
So, once again, science will have to come up with a way to save civilization. And predictably, we'll never get thanked, never get the pay we deserve, never get the recognition, because the "learny-folks" scare those who are ignorant. Oh, they could educate themselves. The common man isn't stupid - after all, most scientists come from the common population. But the average common man is simply unwilling to educate himself. It isn't intelligence that differentiates the public from the scientists - it's the will to learn.