But then what changed between 1900 and 1945? We had two world wars between those years and nothing but small localized wars never between major powers after?

Weapons. We developed weapons that could sanitize the planet, and in the case that someone might challenge our intention to actually use them, we developed weapons that ran the spectrum so that war against us would always seem a major losing proposition. The only thing left is insurgency and proxy wars, always carefully enough to not trigger major action.

Given that game becoming economically codependent and the world is the best play for the available game, and then indeed “peace is good for business”.