Why is war unprofitable? The value of the land in a long run tends towards infinity, especially if it is populated with taxable people and factories.
In the industrial era, the value is the profitable industry, which you just blew up with bombs to win the war, or had blown up loosing the war.
Either way, you spent an insane amount of money going to war offensively or defensively, not winning anything of value other than some barren land which is now worthless, and now you're also broke from the debt you took to fight an industrial war.
Advanced industry means wars are less likely to be profitable, not that they'll never occur.
The US is one big imperial war machine that protects its ownership class assets with their foreign policy, like any other superpower would.
"War profiteering", forceful opening of markets, huge contracts being made after all major wars, resource control?
The forever wars happening from the second world war up until now to keep the West on top. What "we" did to South America, to Iran, to African countries when they wanted their surplus?
Empire logic, game theory, nerdy statistical perspective even if you played a bit of civilisation or read Guns Germs and Steel.
Classical geopolitics 20 years ago was all about these game theories, propaganda, that it's all a big game of Risk, acquiring the most while dominating others like literally all of history.
What is war about today suddenly then? Beautiful philanthropic benevolence of the enlightened western peoples done very reluctantly but with great compassion for the future of the world despite immense expenses?
But maintaining large armies to do such conquering was massively expensive, and consider the extent to which various kings over the years have had their military ambitions crushed because their country or people could no longer tolerate the sacrifices required. The so-called slave labour still has to grow food to primarily feed itself, and this is a fairly full-time occupation in a pre-industrial state. There isn't a lot left over to sustain an occupying force in any sort of comfort (which incidentally isn't farming its home fields when it is doing the occupying).
Wars were often not between states but kings, and most people had no particular reason to favor one king over another. Armies often signaled famine and death, regardless of if they were "friendly" or not. Kings and their vassals usually maintained only small core forces. When more manpower was needed, armies could raised.
War didn't have to be particularly profitable. It only needed to be more profitable than business, which is often was in a zero-growth environment. And it was obviously risky.
The key thing was that nobody wanted a prolonged war, because it was expensive and pointless. Losing was often preferable. Sure the victors would kill some, enslave some, and rob some, but for most people, life would go on as before. Some elites would be different and they would swear allegiance to a different king, but most things would remain the same.
Sure, in principle putting in some fixed amount of resources to get a piece of land forever is worth it eventually. But that's not how it usually works out. If you conquer land from somebody, they don't roll over and accept that. Chances are you will be back on the same battlefield, fighting over that same land a couple decades later. Or if it's large enough empires fighting each other you fight over a different piece of land in a couple years. But either way, the country that can maintain peace without too many concessions and focus on their economy tends to be better off. The only major exception is colonization, where you fight against people who don't have a cohesive country that could retaliate later on.
The cost of war has arguably gone down a lot in the last two centuries or so, since we can now fight total war and have farmers ploughing the field at the same time, due to the insane efficiency gains in agriculture. At the same time the benefits of gaining land have gone down. Doubly so the benefits of gaining bombed-out land where the population was displaced by said war.
Of course, you only control your own choices not your competitors/opponents' choices, so game theory makes the outcome of "everyone chooses peace" nontrivial to achieve.
The taxable people often end up dead and the factories destroyed.
Since about the (Second) Industrial Revolution going after land really hasn't been a good way to gain wealth. Sarah Paine, Professor of History and Strategy at the US Naval War College has some interesting ideas in "The Geopolitics and History of Continental and Maritime Power":