Not quite true - the Americans had significant victories in set battles - not at first, but after getting organized and better funded. Remember the colonies thought themselves more or less autonomous and it was not easy to get them to cooperate.

England had issues with France that were "more pressing" - and in any case, they lost very little in not having the colonies, which had become more or less autonomous by 1760 in any case, to directly supervise. So it is true, they didn't give it a maximum effort - but it is not correct to say they "got tired of kicking our asses and left". Washington turned out to be a superb strategist and fairly defeated the British by skilful maneuvering and tactics when force alone was not enough. These men were not stupid, and they would not have started a war they were not convinced could be won.

A similar argument is often made about the Turks at Galipoli - that they won only because the British were forced to cope with impossible terrain, that the British and Anzacs eventually just said "neener" and split - no, the Turks made a brilliant defense with a force outnumbered 10 to 1 by exploiting interior lines and advantageous terrain.