Now you are starting to sound like some of the Evicons.
The Evicons are a demon faction that always want to wage full scale battle.
But the short answer is: The Eternal Conflict.
Yes, yes to all you’ve said. It’s just not that easy. It’s quite complicated, and demons can’t win by brute force, the gods are too powerful.
Also, realize there are MANY battlefields, Astlan is just one.
On some planes the Eternal Conflict is much more heated and active.