Well, my wild guess as someone that doesn't know much about WW1 is that you might be overestimating how precise artillery was and how large the distance between opposing trenches ?

So "the shells could've been aimed only at the opposing trench, without needing to cover the area directly in front of the advancing soldiers." is not possible because it's the same area ?

Also, I seem to remember something about it not just being two trenches, but several layers of trenches, and also various levels of enemy artillery, so each barrage would have wanted to cover the various layers of enemy defenses, but with different timings, hence "rolling barrage" ??