You know why they think that. They believe, truly, that God made the world for man to exploit and use up, and anyone who says otherwise is spouting the Devil’s lies.
Some will claim to believe that it doesn’t matter one way or the other, that this is the only life we have, and that there’s no point in sacrificing our own joy for the future of others. This apathetic stance is held by many, but it is wielded as a shield and reinforced by those who believe that Earth is just a toy for us to have fun with and break, but won’t say it openly.
Whether anyone wants to admit it or not, this conflict is a religious one, and anyone who denies that is only enabling the wingnuts destroying this planet.