There are three "worlds" in this setting: Heaven, Hell and Midgard, Midgard being Earth and the entire Milkyway galaxy.
When a demon or an angel enters Midgard, they can't just walk there, as they need a body to be able to interact with it. If the body is destroyed, the demon is sent back to Hell with their memories intact.
Turnaround time (Tt) is the time it takes for a demon, immediately after their physical body is destroyed, to create a new one.
Tt varies from demon to demon, high-level demons like Zizel or Lime need hundreds of years, The Icon of John Romero needs a thousand years, "mediocre" ones like Marauder need weeks, however, there's a special class of demons, called Eternals.
Eternals aren't particularly tough or well-armed. Some have rocket launchers, others have shotguns, machine guns, and a handful possess mortars, however, they have virtual turnaround times as low as one millisecond, making them the ideal footsoldiers for invading Earth and holding position. Now, they accomplish these insanely low values by cheating the system. They have multiple bodies pre-made, albeit only one can be active (possessed) at any given time.
However, for the story, this poses a few problems. Of course, Eternals are numerous enough that simply having them active all at once will get you 10 000 units of unique demons to work with. But the main team of the story, the background dancers, have been fighting the same demons over and over again.
Sure, Doomslayer, Sammy and VEGA are leagues above them, so any improvement would go unseen, but the background dancers need each other to survive against the demon horde. This means that the Eternals should show signs of adapting to combat the humans' weapons and tactics, especially since they go up against the same five. It's not like there's anything better to do.
Yet their tactics and moves remain unchanged. They do have intelligence and are able to speak, though you sometimes wish they weren't.
I'm looking for a practical reason. Why do Eternals refuse to adapt to combating specifically humans when all they had to do was to take over that damn planet, CJ!
At the point of the invasion, Earth has modern-day technology.