I think it's a cost thing. I bet those older tanks require a good bit of strength to operate, so to replacing an insulated human that cost a few hundred dollars

I think it's a cost thing. I bet those older tanks require a good bit of strength to operate, so to replacing an insulated human that cost a few hundred dollars a day at most with robotics that can withstand 600 degree operating temperatures and still reliably exerting the forces needed to control the tank would be a tall order and it would take a long time to earn back the equivalent expense