Every account I have read about D-Day and the subsequent Allied campaign in France says that the allied forces were totally unprepared for fighting through the hedgerows used by French farmers in the area to enclose their fields. I understand that aerial photographs were misleading about the height of the hedges and that allied planners were expecting them to be more like English hedgerows, but it is not as if they were fighting on totally alien terrain for the first time. The hedgerows had been there for scores (if not hundreds) of years. The British and Americans had fought in France during WWI. Churchill himself had traveled to France many times in the pre-war years. The French resistance was providing detailed info to allied planners about the location and extent of German defenses on the coast. Yet the first time U.S. troops encountered hedgerows they were gob-smacked by how high and dense they were. Why?
First off, you're absolutely right that the British and Americans (and Canadians, who also participated in D-Day) fought in France in the First World War, but there's a good reason they weren't familiar with the bocage of Normandy: they simply never fought there. The fighting did not get west of the Marne River, and the Marne is well east of Normandy generally, and even more so east of the beaches specifically. They had no experience fighting in bocage terrain.
In addition, though we in North America tend to think of bocage as only hedgerows (many of which have been there for centuries), bocage is actually much more complex. It's the famous hedgerows, yes, but specifically "a series of small fields ringed by earthen banks three or four feet high and overgrown with dense shrubbery” (1) combined with very hilly terrain, wooded areas, small rivers and creeks, and sometimes quite deep valleys.
Aerial photography was misleading about the height, but what was more misleading was how well the hedges (and other obstacles) hid German troops. Bocage inherently favours defenders, and some estimates peg the bocage as reducing aerial recce effectiveness by up to 75%. (2) You can take all the time you like blowing up a hedge or going around it, but the real problem is the 88mm gun or the German regiment hiding waiting for you. Several weeks after D-Day, a June 24th report described the entirety of 1st S.S. Panzer Division and elements of two other divisions as having been able to consolidate almost completely before they were observed. (3)
Onwards to the planning: bocage was accounted for in plans, but was severely underestimated (Carlo D'Este called it one of the "most significant major flaws" of the Allied strategy in Normandy (4) ). Paradoxical as it may sound, planners often thought that a lack of bocage would be the problem. It was believed that, without the cover of the hedges, Allied troops were basically sitting ducks. At Bernièrs (a coastal French town), Canadian troops "were loath to leave [the] cover [of the town] and advance" until naval fire suppressed German response. (5) It wasn't until troops began to encounter bocage in earnest that they realized it heavily favoured defence. You could, of course, always go around it--there are roads, after all. But it was quickly discovered that German troops had been able to aim artillery at these junctions, often shooting from a distance that made them impossible to counter without knowing their exact position (and recall, even with aerial support, the Allies almost missed finding a whole division, let alone one gun).
When troops did break through the hedges, it was often an armoured push, which quickly got separated from the slower infantry, leaving both armour and infantry vulnerable. (5) Patient German troops could also draw units in through or past a bocage hedge, and then trap them with no way to withdraw. (6)
Saying that Allied troops were simply "gobsmacked" it is a bit of an oversimplification. They didn't have experience actually fighting in bocage terrain, and so they underestimated exactly how difficult it was going to be. A large part of that was how high and dense the hedges were, yes, but a much larger part came from how it was being utilized for defence.
Sources:
Carlo D’Este, Decision in Normandy (Old Saybrook, Connecticut: Konecky & Konecky Military Books, 2001) kobo ebook edition, 2005: 179.
ibid 181
Terry Copp, Fields of Fire: The Canadians in Normandy (Toronto: University of Toronto Press, 2003): 79.
Decision in Normandy, 123.
Lieutenant-Colonel T. C. Lewis, War Diary of 7 Canadian Reconnaissance Regiment (17 D.Y.R.C.H.), Textual Records, RG 24-C-3 Volume 14217, Microfilm Reel T-12660, The Department of National Defence Fonds, LAC: 29.
Martin Blumenson, United States Army in World War II: The European Theatre of Operations: Breakout and Pursuit (Washington, D.C.: Center of Military History, U.S. Army, 1993): 41.
Any of these books can give you more insight into the situation about why bocage was underestimated so drastically, particularly D'Este. I'd also recommend Six Armies in Normandy by John Keegan.