The Allies invaded Normandy because it WASN’T as heavily defended as other parts of the northern French coastline.
The German High Command understood that the Allies were coming. They just didn’t know where or when. A pyrrhic mini-invasion of Dieppe in 1942 confirmed to the Allied High Command just how logistically difficult it would be to attack the Germans at their strong points.
The Germans fully expected the Allies to attack at or near Calais, the closest point in the English Channel between England and France. If you think the German defenses at Normandy were formidable, they were several orders of magnitude stronger and deeper in that area.
The Allies understood, too, that no invasion would have been successful without the ability to have supreme air power overhead, which limited the places they could realistically invade. Normandy was close to the outside limit of that range.
The Allies did a magnificent job of subterfuge in convincing the Germans that they fully intended to storm the defenses at Calais, including creating a fictional First US Army Group under the command of George S Patton, the general the Germans feared most and who they believed would lead the invasion. The Allied deceptions were so convincing that Hitler, who had been advised by his spies in England (who’d already been turned by Allied intelligence) that there might be a diversionary attack at Normandy, but that the main attack would still be at Calais, withheld deployment of serious armored reinforcements to the Normandy area for two weeks.
Normandy casualties, mostly from Omaha Beach, were the result of the fog of war, extremely poor weather and, consequently, extremely ineffective pre invasion sea bombardment of German defenses. But the invasion succeeded because the Allied High Command picked the right place, the right time and did their pre invasion homework flawlessly.
