National governments have more power than any other human institution in the world. Why do you think this is the case?

There are many ways of answering this question: I will give you mine.

We live on earth, and space is something we can barely produce more, but which is easy to seize and necessary. So people naturally tried to have land for themselves, and the best situation where conflicts can be avoided is when some groups of people rule over some part of earth independently; this is known as a country.

A country has to be ruled somehow, this is the national government. The national government has to have supreme power in a land, else it risks not being able to defend its borders and a conflict or a war.