Because (in their best indignant, lispy, stuttering academic voice), “the s-s-s-science is already s-s-s-settled!
No one, who knows anything about ‘science’, would ever say “the science is settled”. Science is never settled — that’s its nature.
If climate scientists, collectively, thought that ‘the science is settled’; then, they’d all turn in their grant money and retire — or move on to some field of inquiry that isn’t ‘settled’.