Why did the U.S. choose to assault Normandy by sea?

Ok, so there is actually a lot to unpack here so we ought togo point by point.

It wasnt just the US, why would you think it was? Of the 5 landing beaches (Utah, Omaha, Gold, Sword, and Juno) 2 were assigned to American troops, 2 to the British, and 1 to Canadian units. While a huge portion of the armada at sea moving the invasion force, securing the area, and providing gunfire support was made up of British and allied warships, the UK alone provided about 2/3 of everything afloat that participated, while the landing forces was about 50/50 for US and Allied. It was by and large an Allied effort from top to bottom.
Air support on both a strategic and tactical level was present in huge numbers. Much of Northern France had received a working over from heavy bombers, and smaller fighter bombers leading up to the invasion, seeking to hit targets of opportunity, and limit the ability of German reserves to deploy. While on the day itself in addition to bombardment from dozens fo warships from destroyers to battleships each beach received a strike package of bombers. However for numerous reasons these were less effective than might have been hoped for. The weather wasnt as clear as might have been preferred, bombing runs in some locations overshot and dropped past targets, and German formations adopted moving at night and use of cover when not as tools to protect themselves from air attack. While the strategic decision had been made long before to use surprise to have a quick bombardment to disorient and soften up the German forces, then send the men in, as opposed to the days to weeks long focused efforts seen elsewhere and often in the Pacific.
And for all the mass media depictions do, the landings were in the end wildly successful. While far from every Day 1 objective was in hand 4 of the 5 beaches had been secured relatively quickly and forces moving at least several kilometers inland. While a counter attack from the 21st Panzer Division had prevented the seizure of Caen and the final linking of the British front between Juno and Sword beaches on the Eastern end, the German armor had been thrown back and the beaches were not in immediate danger of being overrun and reinforcements and supplies could pour in. While at Point-du-Hoc the US Army Rangers had silenced the a battery of German coastal artillery before it could prevent the landings of further units from the advantageous point. Only on Omaha was resistance worse than expected, and made worse by the comparative lack of supporting armor. And even then by the afternoon lanes off the beach had been secured, and by evening the process of pushing inland had begun with no immediate threat to being pushed back from German reinforcements. At the cost of only about 3,000 men on Omaha and about 3500 or so across all the other beaches.

Honestly when compared to other landings of a much smaller size thats reflective of great preparation, strong leadership to get off the beaches, and faltering German hesitance. In other words it was almost a bargain for winning a toehold in northern Europe when taken all 5 beaches as a total. And at or below proportionally many other amphibious operations during the war for casualty rate in the landing forces.
We should also note the 3 airborne divisions dropped into Normandy the night before to further hamper German response. In the age before smart weapons, night vision, and helicopters it was very difficult to supply and support fighting units by air. Even increasing the size of the air borne assault it would then have become a race for those units to seize a working port of beach to allow for reinforcements with heavier equipment to land before being over run by the German armored units, with the Allied air forces desperately trying to support them. A FAR riskier proposition and chance of disaster than an amphibious assult combined with airborne landings.

EDIT; spelling mistake