Is the answer not self evident?
That the presumption that 'rules of war' is why men fought in close order is wrong?
It boils down to military professionals over the course of 250ish years werent dumb. They fought in the manner that gave them the best chance at victory as they understood it, using the tools they had. Which were muskets of questionable accuracy at anything beyond maybe 50-75 yards, cannon, horse, and voice/runners/drum/musical instruments to convey information.
So forming men into what amounted to large mobile shotguns that you could actually command isnt the worst thing in the world. It will put out more concentrated fire than infantry in open order or small groups(which was very much a thing!), and can be commanded by a few officers and NCO to coordinate with other groups of a few hundred men towards a common goal in the heat of battle
And in particular we can note that Washington indeed spent most of his tenure in command trying to get the Continentals up to something like European standards. Not because he had some ideas about the 'right' way to fight the war, though he certainly had those. But that is what he thought, and indeed was vindicated in that it gave them the best chance to achieve military victory in the field. A hard core of experienced, trained, Continentals were crucial in the American victories of the second half of the war. But even then both the Americans, British, and German forces engaged in the conflict were not inexperienced in Light Infantry tactics, it was a common feature of warfare both on the Continent and the Americas and a growing one at that!
Some older threads you may be interested in that expand on this topic. And some of the reasons why infantry tactics changed in the 1860's and 1870's. The American Civil War wasnt the only conflict seeing technology and tactics change rapidly at the time!
https://www.reddit.com/r/AskHistorians/comments/29zdx6/18thcentury_battle_formations/
https://www.reddit.com/r/AskHistorians/comments/51unce/line_infantry_tactics_during_19th_century/