The thinktank forecast wasn't optimistic.
Only Mercury, Venus and Earth—the strongest planets weren't facing a big problem.
Because Mercury faced Sin Abyss—their opponents were Beast Awakeners. A few decades ago, Abyssals used parasitic beasts to hide inside the human corpses and assasinated the army officials during cremation.
Thus, it became a tradition to cremate the corpses on the spot.
Venus, on the other hand, faced the Sea Abyss. Since most of the Abyssals were Water Awakeners, Venus recruited a lot of Fire Awakeners to figh them.
As a result, during the war, most of the corpses—Human or Abyssal were burnt and couldn't turn into an Undead.
Then came Earth.
Moroever, since Demon Abyss members were Body Awakeners, they used to humiliate humans by crushing them to pulp. Humans started to return the favor. So, there were almost no Abyssal Undead on Earth's planetoids.