Because we all know it's so easy to chase down fast air units who also have the longest sight ranges and can spot while inside their own air defense umbrella. Oh wait, no, it's actually a great way to lose your choppers and still lose whatever it spotted to the artillery.

It's brokenly powerful. What it doesn't kill it routs or renders combat ineffective and it will be the death of this game if left as-is.

Rocket artillery reloads way too fast.
Regular is ok as you can run from it and it takes ages to readjust.

Just don't group your dudes when you rush.

Heavy/medium artillery has a pretty large impact radius and an even larger morale reducing radius. If you've spread out to the point where artillery isn't going to get a decent number of units in the radius, you're spread out enough that it won't take many units to ruin your "rush" as it arrives piecemeal.