Is this an inability or a deliberate decision? I don't know, but I am skeptical of Russia's ability to carry out any major combined arms offensive, or even anything as simple as orchestrating an armored push through a weak point in the Ukrainian defensive line. They simple haven't demonstrated anything beyond their ability to grind down the enemy through massive artillery fire and extensive infantry assaults (some of which, admittedly, were carried out decently well).
According to Russian pre-war doctrine, in the event of a mobilization they are supposed to use divisions and brigades.
So the reservists fill up the 3 empty infantry battalions of a brigade, and the empty ones of a division.
So far we are seeing the exact reverse, a lot of ad-hoc actions I find.
Instead of brigades and divisions being used in the Ukraine with combined arms like in World War 2, Ukrainian sources are claiming that the Russians are now using 'company tactical groups' instead of 'battalion tactical groups'.
I would like to see what a fully trained, combined arms Russian division and brigade attack looks like.
Further all the Russian training footage revealed since partial mobilization has been unimpressive.