Because the goal of the Russian military is to destroy the Ukrainian military. And, that is happening in the meat grinder in Donbass.
The slow grind minimizes Russian causalities and maximizes Ukrainian losses. And, Russians have been aided in all this by NATO and Zelenksy both of which have insisted on using Ukrainian troops as cannon fodder.
Plus, the slow grind allows the Russians to keep churning out missiles, ammo and weapons all while Ukraine and NATO are running out of all that.
The Russians would “destroy the Ukrainian military” much faster if the Russian Airforce could actually attack the Ukrainian military, where it matters, in its logistics. But they can’t.
The slow grind does not minimize Russian losses. They are reduced to this because they cant do anything else. This is a bad way to destroy an enemy. The idea that Russian casualties are being minimized is a fantasy. For one thing the enemy gets to shoot back, and the Russian artillery and rear areas are a rich target.
The proper, classic military way to do this is to break through, turn their flanks, surround them, or force them to retreat and destroy them on the roads through pursuit and air attack. This is exactly what the Ukrainians have done to the Russians in the Kherson offensive. But the Russians can’t do that.