It's because of the way the issue has been politicized. In people's mind it's a binary issue: either you agree that warming is happening, we are causing it, it is likely to be catastrophic, we can substantially improve the outcome by acting now, and the cost/benefit of this intervention makes sense, or you don't. In reality the basic science is but one small piece, and reasonable people can disagree about the economics or politics of it. But the tribalism of politics make most people boil it down to only two possibilities.
