When flying a plane, it can be very difficult to identify specific things on the ground unless there is an obvious indicator of their specific location.
When the thing you're trying to find is intentionally concealed it becomes even more difficult to find things on the ground.
When you are flying in combat, you have to be focused on flying in a manner that will make it difficult for enemies to shoot you down, and that means you have less attention to spare looking for the targets, therefore it is even more difficult to find things on the ground.
Attack planes in that era did not have large fuel tanks that would allow them to 'hang around' looking for those very, very difficult to find things on the ground. The needed to be able to fly into the area, drop their bombs or fire their rockets on a quickly identifiable target then go back to base for more fuel.
Attack planes in that era did not carry a large quantity of ordnance on board. If you only have enough bombs or rockets to attack a single target, you cannot just go around bombing everything that looks like it might be the target. You need a clear indicator of where the target is so that you can drop that one bomb or fire that one salvo of rockets then go back to base for more ammo.
Those tanks were not the only ones who needed air support and those planes had a lot of work to do. They needed to be able to fly into the area, attack a clearly defined target, then leave so they could go to the next place where they were needed.