Or maybe its because the US has never had to really deal with the dirty work of a war on its territory.
Revolutionary War? Civil War?
Certainly, that does leap to mind--it occurred to me immediately.
To claim the US did the dirty work in WW1 or WW2 is to completely ignore the real facts. The US came late to both wars and didn't lose nearly as many troops as many other countries did.
Well, it is certainly true that our allies did a lot of dirty work themselves, but I think we did our fair share of the dirty work too, at least in World War II (my vague knowledge of WWI precludes my making a definitive comment on it).
In the Second World War, the most of our naval resources were committed to the Pacific Theater. The United States Marine Corps was dramatically expanded to more than 50 regiments in order to fight in the Pacific, and the United States Army and Army Air Forces fought there in significant numbers as well. MacArthur, in the Southwest Pacific Theater, was unable to rely upon significant naval resources from the United States, thanks to the petty attitudes of the Navy, so, instead, he relied upon the slim American resources he got, and the Royal Australian Navy, as well as small detachments of the Royal Navy. He relied upon United States Army and Army Air Forces most heavily, with the Australians close behind, and significant numbers of English troops, and the few Dutch troops available. MacArthur--unlike the Navy charging ahead and slamming head-on into Japanese strong points, and at a horrible cost, paid largely by the Marines--managed to conduct (admittedly in much different terraine situations) a mobile campaign which involved some heavy ground fighting, and a good deal of bypassing of Japanese strong points, leaving huge Japanese garrisons to rot on the vine. The Philippines involved, of course, huge naval and army resources, in the only broad ground campaiging of the Pacific War.
In Europe, the Americans did not even appear until late in 1942, and then came very close to being overrun in Tunisia. To our credit, Rommel in his papers states that he found the American commanders to be very resourceful. He remarks time and again how American commanders would fight until their positions were almost overrun, and then manage to conduct a fighting retreat to new positions, because of what he complained of as the lack of initiative in German commanders. He described Tunisia as the largest self-sustaining prisoner of war camp in history, after Hitler decided to pour troops into a futile defense.
When we landed on Sicily, we fought along side the British and the Canadians. To his credit, Patton used mobility and the considerable firepower of the United States Navy and the Royal Navy, to do an end-run around German defenses. By contrast, Montgomery, whom i personally consider to have been criminally incompetent, spent thousands of British and Canadian lives to hammer his way north on the east coast, and still failed to arrive before Patton did.
In Italy, there was truly a United Nations army. In addition to the Americans, British and Canadians, who represented about two thirds of the force (just about equally divided), there was a huge contingent from India, a South African armored division, a huge New Zealand mechanized "super division," three Australian divisions, three Polish divisions and a Polish armored brigade, a Brazilian division, several French divisions and more than half a dozen French colonial regiments (one from west Africa and the rest from Algeria and Morroco), and a Palestinian Jewish brigade.
The landing at Normandy in June, 1944, less than a year before the conclusion of hostilities in Europe was the first occasion upon which the Americans were to take a leading role. The 4th division went ashore at Utah beach against spotty resistance. The 29th and 1st divisions went ashore at Omaha beach against murderous resistance. The night before, both the 89th and 101st Airborne Divisions had gone in with three PIR (Parachute Infantry Regiments) each, and one glider regiment each, along with artillery brought in by glider. The English landed at two beaches themselves, and met spotty resistance, but which was murderous where the Germans did put up a fight--the French went in with them with special forces. The Canadians landed at their own beach, and met little resistance, until they drove inland. Then their casualties were horrible, and they became involved in a six week nightmare fighting the Hitler Youth SS Panzergrenadier division and an SS Panzer division.
Gradually, the Americans reinforced to the entire First Army. After La Falaise, and the escape of the Germans through the "gap," Third Army was landed, and joined First Army in Omar Bradley's Army Group. Patch landed in the south of France with Seventh Army, many of whom were veterans of hard fighting in Italy--which while it substantially increased the American forces in France, greatly reduced their presence in the United Nations army in Italy. Montgomery's Army group was built around Horrock's XXXth Corps, largely armor and mechanized troops, with the Canadians and the American Ninth Army. The Americans did not reach a superiority of force among the allied in the west until there was far less than a year left to go--at no time did the combined forces of the Empire and the United States reach the proportions of the Soviet ground forces committed in their "Great Patriotic War." The large numbers of Australians who had fought with the English in Africa had been greatly reduced, as a Labour government insisted on their return to defend Australia--when they joined MacArthur.
None of that is intended to minimize the importance of the American war effort--but it is to point out that the Pacific was the focus of our efforts, and despite Roosevelt's desire to "get Hitler first," a dubious proposition advanced against him by revisionists, the most of our resources were used to combat the Empire of Japan.
In the Great War, the arrival of the Americans was certainly crucial--nevertheless, the fewer than a quarter of million casualties does not even remotely stack up against the more than a million lost by England, nor the losses of the French, who lost a million casualties at Verdun alone. The Canadians, with only a population of 7,000,000 in 1914, became, along with the Australians, the "shock troops of Empire" as the English military dimwits liked to call it. Somewhat more than 600,000 Canadians participated in the European war in 1914-18, and more than 60,000 of them were killed outright or died of wounds. The Australians suffered a similar proportion of loss compared to their population. For the Americans to have matched the Canadian commitment, we would have had to have dispatched more than 8,000,000 troops, and to have suffered more people killed or died of wounds than the entire butcher bill of the American civil war. But our casualties were not even a quarter of that figure, and we only had a million troops there for the late 1918 offensive against the Germans--we raised that to 3,000,000 by 1919. There is no doubt that we arrived in the nick of time in many respects. There is also no doubt that we did not suffer anything like the human holocaust which the Europeans visited upon themselves. In the Second World War, all Allied casualties other than Soviet combined did not make a siginficant fraction of the Russian losses.
Some reasonable perspective shows that although our contributions in both wars were crucially important, we did not come anywhere close to having "bailed them" out in Europe on either occasion. It has been alleged that without the Americans, the German 1918 offensive would have succeeded. I doubt that based on the evidence. They (the Germans) might have done better than they did, but they simply did not have the logistical or human resources to "break through to the sea," or to successfully drive on Paris. Our landings with the English in 1944 largely simply assured that the Soviets would not drive on to the Bay of Biscay.