That is a good point, the only thing I can think of and it might not even be a good point is that the supply chain for drone manufacturing is significantly higher tech which makes it possibly more difficult to maintain in a longer conflict. When drones get junked they may be impossible to repair where as a tank is a mechanical contraption that can be fixed in the field. Thinking it out I’m sure iron, oil and munitions are all huge barriers too so I don’t want to hand wave that away either