You can act together in formation without being in total lockstep for every motion. Did all the infantrymen swing their swords at the same time? Of course not. It doesn't make any sense for one archer to "wait" for their neighbor before firing. And as the article points out, firing in unison would increase fatigue, not reduce it.
I wouldn't be surprised if it also makes it easier for the enemy to respond. If arrows are only coming in concentrated volleys, then it might be easier for the enemy to make a coordinated advance where they just periodically crouch behind their shields as a volley comes, and then make relatively unhindered advances during the lull.
A diffuse but more-or-less constant stream of arrows arriving at random is presumably more distracting and harder to deal with, and therefore a greater hindrance to enemy infantry.
I was surprised not to see this reason mentioned in the article. It was the first reason that came to mind, especially with popular depictions of the Roman legion's turtle formation.
It does make sense. You don't want archers to burn energy & ammo by firing too rapidly or before the enemy is in range.
I agree that sitting there for minutes holding the bow taut is dumb. But something like a Attention, Nock, Ready, Draw, Loose cycle isn't that. Maybe you hold it a half second and it's slightly more fatiguing but I don't really see it.
If it is you just combine draw and loose. It will make the spread between arrows slightly larger but it's still a volley.
