Within nature, which is the true timeline of our existence, nothing lasts forever. The answer lies in economics. Natural objects which can be easily gathered have less economic value to our society then those items which must be purchased. And, whereas the art works can be resold for healthy profits, the best that can be hoped for the natural materials is to be able to donate them to a museum once a significant number of them have been assembled. Essentially, if society can't put a dollar value on it, it lacks merit.
Art created with and in nature has a very different sensibility however.
Humankind has long disregarded the value and importance of the natural world. We bulldoze over, and pave and push nature into little squares or parks so that we can find our straightest line between two points. We have consciously disassociated ourselves with our natural origins. If we can't process it or profit from it then we don't value it. Of very grave concern is the environmental degradation and pollution which we have imposed upon our natural world.