My Idea:
INF:
Stats are focused on "Defensiveness" , Pure INF should be extremly terrible in makeing an offensive. But pretty good to defend things.
So if we have 2 Pure Infantries army face each other, it ends in a stalemate. You need to out maneuver, massivley outnumber or let the enemy run out of supply to win.
ART:
stats are focused on "Soft Attack" , Artillery allows to deal casualties to the enemy makeing the manpower grind more efficent for you, potentially enough to push the enemy back.
Planes and tanks can replace ART in its function of dealing casulties.
ARM:
stats give good soft (+hard) attack, armor and toughness. Tanks allow to make an offensive without suffering too much casulties, achieving a breakthrough.
An army haveing access to tanks will be able to breakthrough the enemy line and encircle the enemy.
Two armies with tanks faceing each other will end in a war of maneuver.
An Army without ART would work. They are not an automatic choice. But if you have no access to Tanks or Planes, it is the only tool left to make an offensive.
But is that not how it works right now? I don't know the exact stats, but I'm pretty sure that artillery has superiour Soft Attack and Infantry Defeensiveness + Organization, which will lead to what you describe. I don't really see what's so problematic with how things are working now.