I guess that supporting a decent sized maneuver force to a significant operational depth might be beyond the logistical capabilities of either side at this point. When the Russians tried early in the war, the presence of large numbers of ATGMs also made this pretty dicey.
Punching a battalion sized armored group through the front lines somewhere is probably doable, but what next? Without infantry and artillery support, that armor is toast but if you bring everything else along too then you've just added a much bigger strain on extremely limited logistics.
Both sides seem to be having to feed reinforcements in piecemeal too to replace casualties which I guess makes it harder to amass any real operational reserve.
The defender also gets to take advantage of interior lines and an extensive rail network to bring reinforcements to a threatened sector. It really is like WW1 all over again. With both sides short of manpower it's becoming more and more of an artillery duel too - Western quality versus Russian quantity.