While Zayn's progress and the expansion of the Spirit Army were exceptional, to the point where it didn't make sense at all, there were considerable drawbacks as well.
Two weeks after the Natural Selection War began, the Demonlings destroyed one of mankind's shelters. Shelters were like indestructible fortresses that were spread across the border of mankind's territory.
They had been constructed through the sacrifices of millions of powerful Blessed and Normies at a time when all humans had been forced to work together as they didn't want to die miserably.
In an era full of pain and suffering, mankind worked together. The shelters were the best evidence of mankind's power and what they could achieve as long as they worked together. Such a shelter had been conquered under the constant attack of more than four million Demonlings.
Their rank might have been lower than the strongest defending Adventurers, but their numbers made up for the lack of power.