According to world-systems theory, why must the global economic system be approached as a single capitalist unit?

According to world systems theory, why must the global economic system be approached as a single capitalist unit? That is because the current system of world capitalism is not a collection of independent countries.

World-Systems Theory is a (post-marxist) theory of international relations, geoeconomics and international political economy that focuses on the study of the social system and its interrelations with the advance of world capitalism as determining forces between the different countries (based on the concept of world-economy, invented by Fernand Braudel).

The global economic system is the system of production, distribution, and consumption of goods and services of various economies.

Note: The capitalist system is an example of an economic system.