User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-24098614-20131226024537/@comment-16281343-20131226074826

That is the for the setting of the story. -_-

Normal birth rate = No Evil Pieces = No Highschool DxD.

And also,

Long life span + "normal" birth rate + no apparent danger that can decrease the population = overpopualtion in the territory of the Devils in the Underworld.