When the guns were so inaccurate that getting more shots was more important than actually aiming, that's the natural tactic to employ.
Lining up was hugely important for other members of the army to know what you're doing. If you're 1st in line, you shoot. Back of the line you're loading gunpowder (with everyone else). Middle of the line, you're dropping the bullet into the musket. Etc. etc.
Like a choir performance: its more about having tons of other people doing the same thing you're supposed to be doing. So no one in the army ever "misses" a step in the ~1-minute reload process.
Even if you're not that good at singing, you get "better" in a choir because you just follow the crowd. Similarly, even if you've forgotten a few steps in the long reload process, you just mimic the actions of your neighbors.
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Note that the Redcoat beat the US rifles at pretty much every engagement. It was guerilla warfare that allowed George Washington to survive and last through the war.
Rifles took longer to load. Without precision machining, the bullets needed to be wadded up with paper to stick with the rifling, adding a laborious step in the shooting process. Muskets could fire more shots per minute due to the smooth (though inaccurate) barrels. In army vs army battles, aim doesn't matter, your bullet will hit somebody over there, so there's no advantage to the superior aim that rifles have. You've added 20-seconds to your reload process for almost no tactical advantage at all.
USA experimented with rifles / guerilla warfare, but it really wasn't that successful in the 1700s. (It got the job done but... George Washington lost far more battles than he won). Even in the mid 1800s, the British Square formations were doing decently.
