Every inch of the front and the hinterland is permanently watched by both sides via drones.
Are you seriously claiming there are enough drones to have full surveillance not only over the entire front but also the lands around and far behind it?
Drones have had and have a significant impact. But they are not that numerous and covering - that's not feasible nor even physically possible (resources + products + management). There's no "permanent drone watch".
Notably, drones are used for more than just observation. They are used for targeted strikes, dropping shells, or drone-suicide sabotage strikes.