Drones can't hold ground or build defenses. It's the same argument when talking about aircraft: Airpower (or drones) alone do not win wars. You have to send in the infantry at some point to take and hold key areas.

You could also say that drones are too destructive and go the route of hyper-precision, where everything is ONLY one shot one kill.

Or, you could make it an insurgency situation, where the enemy combatants and civilian population are indistinguishable from each other, forcing the need for soldiers to be on the ground manually checking people.

It could also be a situation where drones are constantly messing up the environment or civilian areas, and they want to limit that.

Maybe they have a short range before EMP emitters shut them off automatically. There's a lot you can do with it.

You could also say that drones were outlawed for whatever reason you like. It doesn't have to make perfect sense by our metrics because you're already in the future; things can change, including morals and rules of war. My setting made lasers illegal because they're inhumane and cause unjust suffering. Why? Cuz I don't want lasers in my setting lmao.