I don't want to particularly delve deep on specific theories of international relations but, a commonly notion held by realists, rationalists, idealists and other schools of thought lying outside and within these primary schools is that great powers do not fight each other. The explanations as to why Great Powers at least in post WWII world do not engage in full blown wars and resort to low level proxy wars are many but, one of the more popular explanations is that wars between great powers is far too costly. Costs in this context can range from human life, infrastructure, resources and even global political image. This particular explanation is very in line with the One Piece World and the maintenance of the status quo between the Great Powers.
The Ace novel elaborates on the power dynamic in the world and why the Great Pirate Crews do not engage each other in an all out war. The answer to this is because it is simply too costly. It is important to understand that the Great Pirate era is far different from the old Era in that individual power and combat strength means far less than in the bygone Era. In this Great Pirate age, the strongest in the world cannot freely conquer the world or showcase their strength because other Great powers and pirates serve as checks and balances. What these checks and balances mean is that if a full scale war is to occur between two Yonko crews, the winner of the conflict would have taken immense loses in all types of resources. The power balance exists because these Great Pirate Crews balance one another and are in perpetual conflict.