I watched saving Private Ryan recently, and something that struck me as absurd (being a veteran myself) was the obvious presence of major defensive capability by the German army. Why didn’t the American and British navies shell the absolute bejesus out of the landing sites before deploying ground forces?
Well, the first element is that Saving Private Ryan is a film, designed for entertainment rather than accuracy.
The second element is that there was substantial efforts by the Allies in terms of both naval and air bombardment to neutralise strong points, but--given the limitations of air support and naval artillery fire at the time--even the heaviest bombardment was limited in its ability to permanently destroy defensive position. /u/thefourthmaninaboat goes into great detail here on the activity of Allied battleships before and after the landing. /u/badgerfest also describes the limitations of airpower in destroying strongpoints here. To wit, there was substantial bombardment by both air and naval forces, but the limitations of these actions meant that even these prolonged operations would not have resulted in simply strolling onto an abandoned undefended beach.
Even in the Pacific, where the Japanese fortified islands with far more limited resources than the Germans could bring the Normandy, and the United States could--and did--bombard islands for days at a time, Japanese fighting positions would often survive and be operable as American troops landed.
Simply bombarding fortifications is unlikely to result in their complete destruction. Their suppression? Yes. But complete destruction is highly unlikely.