The very short answer to this is that winning battles is not the same as winning the war. The battles he lost did not destroy his army ( though the early campaign in New York, especially the Battle of Long Island, did come close) and he was able to keep a force in the field, with very uncertain sources of supply, and remain a threat to the British control of the colonies until Yorktown. The fact that he was mostly sparing of his troops, did not waste them against overwhelming odds, also helped their high opinion of him. He could have been otherwise. As someone once said about U.S.Grant, as a soldier serving under him you won, but you were dead.