As mentioned, war is a billion dollar industry. But also, international law is complicated to enforce. Countries aren’t so easy to control that you can just fine them unless there is a greater consequence for not doing so. That greater consequence is often war, or some form of sanction.
But you also have nuclear powers in the mix, making war an ineffective method of enforcement, because the risks of war between two nuclear powers is greater than can justify whatever enforcement caused it. Leaving the next best tool to be sanctions. But not every country will honor those sanctions, and you create these crazy little countries like Iran or North Korea, who build nuclear weapons and cozy up to your enemies instead of you.