Y-bar gave an excellent reply, but further to that the fact is grave robbing and raiding for profit is a thing that happens. Vast amount of historical information and material has been lost this way already, and more is lost every year. Leaving things in the ground is no guarantee that they will stay that way. There's an argument that by excavating this material respectfully, gaining as much information as possible and preserving these artefacts is better than abandoning them to eventual robbery and destruction.