Because the alternative made even less sense.
Stalemade on the western front was because of A) the German attack bogged down giving the French time to entrench. and B) Because neither side was willing to violate swiss neutrality.
Both sides had sufficient troops to prevent any flanking of the front resulting in expensive offencives to push back the front and never achieve a breakthrough until the advent of armor.
The front with Georgia was for about 4 months this exact kind of stalemate until i realized Finland was not neutral afterall.
Point granted on the Finnish front - if a front simply becomes too long for trench warfare, then it's too long. But I do disagree with your analysis of trench warfare.
In WWI, despite the name, the stalemate of trench warfare was not really due to trenches, literally. It was never impossible to break through the trench lines, it was impossible to exploit breakthroughs, because the defender (on whichever side) had a huge advantage in communications and logistics and so could move and concentrate troops much more effectively than the attacker. The massive trench lines were just a better way of defending that arose naturally once warfare had
already become static due to the defenders natural mobility-and-concentration advantages on the Western Front. Note that in 1914, first the French stopped the German attacks,
then the trenches started growing, not the other way around.
Armor didn't change this equation at all, nor did other breakthrough weapons such as stormtroopers or simply massed artillery. They just made it easier to do what was already possible, break the trench lines, but they didn't help with the real problem of the Western Front - that the attacker was unable to move and concentrate troops better than the defender irregardless of the trenches. That's why the war in the West ended literally when one side was too exhausted to continue, rather than on any of the many times when offensives temporarily broke the front lines, whether with tanks, artillery, stormtroopers, or just mass human waves.
I'm curious because the stack rule you've implemented would seem to give the mobility and concentration advantage to the attacker, when really, it was the defender that enjoyed these advantages (assuming relatively equal infrastructure, etc) up til the advent of motorized troops and the widespread military use of radio. But I guess it's a somewhat academic distinction for Vicky?