It might be the situation more than the kind of people we are. For a steppe nomad, the easiest way to get rich was to take stuff from the Chinese. For a noble in twelfth century Europe it was possible to gain power and prestige by taking control of the holy land. Now what do we get if we conquer a nation?

Imagine tomorrow all of the EU decides to go and conquer Madagascar. Madagascar would lose that fight, but then what? Pay for an army of occupation? Could we even extract the resources from the people to break even on that? Kill all the people there and have fun with the empty land? Wouldn't it be cheaper to just buy the land we want the legal way?

Then perhaps we go some place richer. Japan is rich, let's pretend Europe tries to conquer that. I don't know about the odds of winning, but it seems pretty certain that millions will get killed in that fight. Japan could bomb cities to smithereens if it had to. After all that blood was spilled, and if the EU won, there would be the same problem as in Madagascar. Now it seems possible that Japan is rich enough to make an occupation profitable, but I'm not sure it is the best investment we could make with our national capital. Perhaps we could educate children, invest in infrastructure and stimulate innovation without a war with Japan to achieve higher returns.

Then there are the big countries, taking those out might give so much global market share to gain that it would make a real profit. But places like China and the USA have nukes, so I guess that would end the species. That doesn't seem like a good idea either.

Maybe developed countries today have more to lose than to gain in wars of conquest. All of my speculations assumed we don't give the people who are to be conquered basic human rights, this might be a very bad idea in and of itself, since it would make it easier to take the rights of citizens too.