The military rank of field marshal is the highest general officer rank for most countries that have land forces. Traditionally British monarchs hold the rank, even though Her Majesty has never officially assumed it (no real need if you are already Queen, Commander-in-Chief and Head of your Armed Forces). In times of peace, the rank is usually the preserve of senior royals like the Duke of Edinburgh and the Duke of Kent, although there have been some notable exceptions over the decades. Extraordinarily, the Duke of Wellington was field marshal in twelve different armies. Only one American can lay claim to the rank: General Douglas MacArthur was appointed a field marshal of the Philippine Army.
Field Marshal Marshall doesn't exactly roll off the tongue
As for why America never adopted the pinnacle rank, one may be forgiven for thinking it was thanks to George C. Marshall (see photo above), the very first 5-star army general (field marshal equivalent) of the United States. Field Marshal Marshall would have sounded undignified, and leads immediately to the theory that President Roosevelt out of deference to his chief military advisor (in Churchill's words, the "organizer of victory", the man who, after all, expanded the size of U.S. military forces forty fold), instead proposed a new rank to Congress called "General of the Army". The rank was duly approved and General Marshall was promoted on December 15, 1944. (General MacArthur received his promotion the following day, not because he was less senior to Marshall, but by virtue of not being in Marshall's shoes as Army Chief of Staff, the same position that MacArthur held more than a decade earlier.)
Indeed the need for American field marshal status came to a head in September 1944, when the irascible general, Bernard Montgomery, was promoted to the highest rank in the British Army. How could (4-star) General Eisenhower carry out his superior function as combined Supreme Commander in Europe, if he was of lower rank than Field Marshal Montgomery? As noted, the issue was solved by an act of Congress in December 1944, and Eisenhower became a 5-star General of the Army one week after MacArthur.
General MacArthur for his part, was later considered for an unprecedented new 6-star rank, "General of the Armies", which planners were calling for in July 1945, given the sheer scale of the invasion force being contemplated for Japan and its surrounding islands. The Institute of Heraldry produced a single sketch of how the insignia for six star rank would appear, which was later filed into Douglas MacArthur's service record. The 6-star rank was not to be, however, and the Instrument of Japan's Surrender was signed on September 2, 1945. Interestingly, the United States came within a hair's distance of establishing a military rank superior to that of even a field marshal.
That said, the senior 5-star (initially a 4-star) rank of "General of the Armies of the United States" (a development that goes back to 1799, which is the substantive reason why field marshal was not chosen) does currently exist and is the highest possible officer rank of the United States Army. Only two soldiers have been granted the rank of General of the Armies; John J. Pershing in 1919 to honor his service in World War I, is the only person to be promoted in his own lifetime to such a rank; and George Washington (a retroactive Congressional edict passed in 1976 promoted General Washington to the same rank but with higher seniority), as part of the American bicentennial celebrations, to commemorate his leadership and involvement in the founding of the United States. As mentioned above, Douglas MacArthur was considered for the rank, both during and after World War II, but a formal promotion order was never issued.