Because people are brainwashed, that's why. They see everything natural as a "problem" and something to eradicate rather than encourage.

I'm in the process of putting together an educational event for my city to educate people on the cooling benefits of trees (and to varying degrees, other plants). It's for the private citizens, but if I get an opening I'm going to try to convince the city council that they would benefit from planting trees along the south/west sides of city buildings and lining parking lots, because I feel like urban heat control is an issue people can get behind.