What you need to understand is that the Normandy Landings were not all like the first 15 minutes of "Saving Private Ryan". The D-Day planners underestimated how well-defended Dog Green sector of Omaha Beach would be, but in other places the Allied troops came ashore virtually unmolested.
The planners did make preparations for the landings by launching a huge aerial bombardment of many if not all of the sectors of the Normandy beaches, but precisely hitting machine-gun nests and artillery emplacements with medium bombers flying at 10,000 feet is basically impossible without guided munitions. The USAAF and RAF bombed the beaches but couldn't have out all the defences. For a couple of hours prior to the landings the Allied flotilla provided heavy artillery bombardment on the shore facilities, but again, they couldn't be expected to take out every German position.
Also, battlefield communications were still far slower in 1944 than they are today. The commanders at sea or in England didn't know that the first waves to land at Dog Green and Charlie sectors of Omaha were taking 50%+ casualties until the evening of the first day, by which time the Germans defending those sectors had either been overcome by the Allied troops reinforcing the first waves, had retreated or had run low on ammunition.
Finally, had there been air support allocated to the beaches at the same time as the landings, the risks of friendly fire were too great. Later in the campaign, at the beginning of Operation Cobra, hundreds of Americans were killed or injured when they were hit by their own bombers which were supposed to be destroying the German defences in front of them.