Because they worship things other than God. Environmentalism is the religion of those who don't understand the nature of man, earth and their co-existence. People like Pelosi believe we are some kind of parasite just here to destroy the planet and we must be stopped...unfortunately this unbelieving Catholic doesn't read her own books. The earth was provided for man, by God, and man was given stewardship over it and everything therein. Yes we are to care of it, but it was created for our use. God created a planet that is self sustaining until he himself decides to take it out of existence. Nothing we do will change his plans.
Man was not on the earth when the ice age occurred, nor were we here when the dinosaurs died out...there is little we can do to stop the planet from warming or cooling...it is just man's small mind that he is bigger than God if he thinks he can change the course of the earth. If Environmentalists were intellectually honest they would find that most of their "theories" and their attempts to control nature actually make a bigger mess of things than they clean up. There is nothing wrong with making clean technology and abandoning dirty ones, there is nothing wrong with trying to limit the pollution we expel...but I find it crazy that we are listening to pundits who have bigger carbon footprints than anyone else preach to us how to save the planet.