They thought they would face tired Germans like they did in France.
Did you know why Operation Overlord worked so well? Hitler was concerned that the Allies would attack at the Narrowest point of the English Channel so he had more of a build up there. Plus the 81st Airborne and English Paratroopers got in behind enemy lines and soften defenses. Also the Allies had tanks they were going to roll in from the water but the water was too rough and some of the tanks sank. Which was also the inspiration for modern day tanks and IFVs can go from Sea to Land
You also forget a major detail: The US had planned this along with the British for at least 2 years and attemp to make the German belived they will attack at...well, i don't remember where exactly, but they did all they can to make the german belived that the Normandy will just be a diversion. So the German would thought that Normandy is not the place to be worry about.
On the night of June 5 to 6, 1944, Airborne troop from the US and British Army have landed behing enemy line to cut their line and weaken their artilery. Then, at the morning of June 6, 1944, the US/British/Canadian troop land on the beach of Normandy in a attemp to free France.
At this point, it was the second time the Allies try to liberate France for the Nazi. But this time, it succeed!
Trivia: When the German officier heard about the landing troop at the Normandy, Hitler was sleeping, so no one did want to awake him. Later on, when Hitler was informed of what was happening at the beach, he was sure it was just a diversion (prooving the plan made by the US/British had succeed).