Also with historical bows you can't ready and hold a shot, you have to draw and release in a smooth motion. Modern bows use pulleys with weirdly shaped disks to reduce the force required to hold it when fully drawn, but without that innovation holding an arrow at full draw would quickly exhaust even a trained and fit archer.

What this comes from is blackpowder warfare where groups had to synchronize their fire in order to hit anything. Especially when muskets were smoothbore before the invention of rifling.

And even there it's more about keeping ranks synchronized, since they were standing in a tight formation and needed the preceding rank to be kneeling to fire over them. You can't have people working at their own pace when they have to act during a certain window of opportunity and then get out of the way and start preparing for the next volley.

Their military theory also emphasized the volley as something terrifying and withering, that a sudden barrage of musket fire would be more likely to make a charge falter or rout a line between the noise and having a large number of simultaneous casualties all falling at once.

Oh, and while we're on this, there's a common myth about line infantry being "unwilling" to shoot at the other line, and so firing into the air above them or simply not firing at all, but the actual historical evidence is that a) muskets were unreliable as hell and would frequently misfire, and because of the nature of volley fire soldiers would commonly just not notice that their gun didn't fire and try to reload it again, and that b) contemporary sources observed that soldiers would overestimate bullet drop over distances and try to compensate by aiming higher on the target, and that they further weren't aware of how much recoil would jerk the barrel upwards before the round left it. Meaning they'd aim for the head or upper torso of someone 100-150 yards away, then the recoil would send the muzzle up and the shot would miss by a foot or so. Contemporary sources include recommendations to train soldiers to aim for the legs of the opposing line, on the assumption they'd still aim higher but would keep the margin of error within the height of the target at least.