They might have learned that they get sick more when dead bodies are left lying about. If afraid of death, they may have wanted to have few reminders of it. They might have thought burying protected the living from the dead which may seem to imply an afterlife but not necessarily. They probably didn't want scavengers attracted to the camp. If the dumping place was far away, they probably yet thought animals eating their dead would develope too much of a taste for them & may be more likely to attack. They may have been repulsed by the possibility of anyone stumbling onto the bodies.
