And perhaps American staff planners had some reservations about British planning and tactics. Almost everyone can agree the British high command squandered lives shamelessly in WW1. Remember the old military saying, 'Generals are always fighting the last war', US planners may have had the criminal waste of manpower in France a mere 25 years before, in mind, and were shy of using British plans.
IIRC the Brits were notably less aggressive than the Americans, taking a month or more to take Caen. This may have been repeated by some of their units (not the airborn troops) in Market Garden.
But, after five years of war they were actually running out of combat-age men by that point.
