Looting can't coexist with discipline, it makes soldiers ignore objectives and being more interested in how to transfer questionable goods back home. It's suitable for barbaric raids of a hit-and-run nature of the past, but in a long multi-layered offensive it means soldiers on the ground has low morale and act on their own, that renders their part of the front vulnerable. Modern armies make sure their troops have food, fuel and other resources for them not to ask around and be easy targets for a sabotage or just hunger, and looting is even worse than that.

Today, no looted goods can cover the price of the offensive. The land taken is useless if it's underdeveloped or\and littered with mines and metal. Human resources aren't effective as uneducated slaves you can't trust as it was long ago.

Modern warfare is political and economical. Looting is done by making other party depend on you, work for you, without hurting their own productivity. Let them be happy to take loans for your people to build them infrastructure, mines, oil rigs, provide security, pocket their politicians and get on boards of all important corporations. Choke them with a belt to make a road. That's when you order to shoot any individual anachronic looters on sight for they mess with your bigger picture.

Doesn't make it any more ethical, but shows how outdated some doctrines may be.

Maybe in 1637. These days, the Wealth of Nations is no longer primarily derived from things which can be looted.

Well I mean it is a war crime but also yeah it’s also kind of the point

Anders Puck Nielsen released an interresting video, where he argues that warcrimes are rusky way to challenge the current world order.