The Wehrmacht was an offensive military force. Its successes relied on blitzkrieg (lightning war surprise attacks with overwhelming numerical superiority) and deep thrusts by Panzergruppen. They were ill prepared for a prolonged siege OR a defensive standoff.
Additionally, both the Germans and the Soviets used a scorched earth policy during the initial drive on Moscow. Unless they retreated to pre-Barbarossa lines, there wasn’t anywhere the Germans COULD establish fortifications and hope to have adequate means to feed and resupply their armies.
Finally, Defensive or offensive operations would not have mattered. German was running out of oil. Rommel couldn’t let his tanks romp in Africa because of oil rationing. The Luftwaffe was essentially grounded by the time of the Normandy invasion because of the oil shortage. The whole point of Operation Barbarossa was SUPPOSED to be seizure of the Caucasus oil fields. Hitler changed the plan on the eve of the attack to send the brunt of his forces against Moscow and Leningrad.
Which is why the eastern campaign was doomed regardless……especially with all the resources being poured into the "Final Solution."