[bunderbunder]

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.

[mitthrowaway2]

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.