Maybe because during ten's of thousands of years of WARRING with each other, every time the dead were left to rot, entire populations were wiped out by a PLAGUE.
Or maybe they wouldn’t give a crap about plagues, especially when fighting in dry desert air (likely to kill bacteria-which they had limited understanding of, let along that it can come from rotting bodies), I doubt an victorious army would take the time (in most ancient cases) to bury their vanquished.