It's because in many cases of endangered species, we are to blame. If they became endangered naturally, you'd be right to not intervene and let them die out. However, for Polar bears, whales, white tigers, rhinos, elephants and countless other animals, we caused their demise in many ways (global warming, hunting for delicacies, skins or ivory) We're responsible for nearly killing them off, so we should be responsible for at least keeping the remaining ones alive.