In addition to all the stuff Dachs said, bear in mind the attack was a complete tactical surprise as the Germans believed the real invasion would come at Pas-de-Calais and the Allies actually engaged in a ridiculously extensive counterintelligence/deception campaign to feed this false belief. This was so successful that even when the landings in Normandy actually took place the German high command (not just Hitler, though of course most of the surviving German officers claimed that it was all Hitler's fault) believed that the attack was a feint and the 'real' landing would come at Calais. Many of the German officers in northern France had also left their commands to go to the war games at Rennes, and so spent the morning of the 6th travelling back to their units, many having received word of the invasion while on the way to Rennes.
Eh, the value of the Calais deception operation - and most other Allied deception operations - is probably overstated. The Germans placed the largest and best-equipped formations and the best defenses in the Pas de Calais department because it was the shortest plausible route toward the Reich. It was a high-risk, high-reward avenue of approach, the sort of place that they themselves would've attacked were they in the Allied shoes. Sure, the phantom First US Army Group fed this preexisting delusion, but did little else. And the Germans kept 15. Armee there even after the Normandy invasion because the threat of a second Allied landing was actually quite realistic - after all, the Allies
did mount a second landing, Operation DRAGOON, in southern France. The decision arguably ended up working out for the Germans - 15. Armee did not really take part in the futile struggle for the Norman
bocage country, and was not bled of vast quantities of men and equipment. Instead, it escaped the jaws of the Allied encirclement, retreated to the Low Countries, and forced the British into a long, slow, bloody fight in the autumn.
While many German officers were away from their posts on the morning of 6 June, this probably didn't end up mattering all that much, either. German military doctrine was, after all, based on the independence of the subordinate commander,
Selbständigkeit der Unterführer. Dollmann, Marcks, and the others might not have been physically on the battlefield, but their staffs were capable of working out what they needed to do. All of the decisions that could actually affect the beaches on 6 June were taken regardless of the presence or absence of the individual general officers responsible for given formations. All of the units that could plausibly have affected the outcome on D-Day
did take part in the fighting in some way. The biggest limitations on German action were not the commanders but rather the fact that there were just so few of them and just so many of the Allies. German armored counterattacks failed to materialize not because Hitler slept in but because Allied air support, especially the feared
Jabos, kept the Germans from moving very far in daylight. The Americans were not swept into the sea at Omaha Beach because the Germans simply didn't have any maneuver formations to do that job.
I always figured that even if the Germans had successfully gotten their poop together and thrown something like an armored corps at the Allied beachhead before the 6th was over, the fleet would have broken up the attack almost as soon as it got going. Putting tanks on the beaches would have been an invitation for them to get smashed at, basically, point-blank range with naval artillery.
American firepower repeatedly smashed German armored counterattacks on the beaches in the Mediterranean. The Hermann-Göring-Panzer-Division at Gela and XIV Panzer-Korps at Salerno both collapsed due to US Navy gunfire. The Americans and the British were ready to do the same thing in Normandy. Normandy was a much less even contest than the Italian battles, with a
lot more ships and a
lot fewer Germans; they would've gotten pasted.
The Germans, of course, didn't
have a Panzer-Korps to hit the beaches on 6 June. They had to spread out their armor across France to account for the various possible Allied avenues of attack, and there wasn't enough of it in the first place. That left them too little to smash the invasion the day it came. And Allied airpower meant that the Germans would build up for a
later counterattack much more slowly than the American and Commonwealth forces would - in sharp contrast to Italy, where the Germans were able to mass large armored counterattack forces before the Allies could effectively build up their beachheads at Salerno and Anzio.