Anti air defense exists and artillery has limited range, so you're potentially risking the life of a fighter pilot, which is arguably the single most valuable position of any country's military or risk hundreds of lives pushing the front forward to allow the artillery systems to deploy within striking range. While that's happening the armor could be moved or even worse, the areas within range are already zeroed by enemy artillery and when you deploy the artillery systems, shells start raining on your position.

Given these two options, its far more responsabile to force your enemy to mobilize such a large invasion force into strategically predetermined routes where artillery is already zeroed in, drones can lie in wait, mines have been laid, and the complex logistics of assault supply can be distrupted.