As I understand it the theory is the AEs noticed the perfect preservation of
natural mummies, preserved by the hot sands and, when they discovered more elaborate
burial caused decay sought to imitate the effect artificially.

It is well established by texts that an intact mummy was considered important if not
vital to the spirit's survival. Elaborate measures were taken to insure fallbacks should
something happen to the mummy; 'spare heads', whole statues, magic formula, etc.