Why do historians call the cities of sumer "city-states"?

Sumer is the earliest known civilization in the southern-Eastern Mesopotamia region.

During the bronze age, the area was known for the developed of anciest villages, cities and 'city-states'

These cities were called 'city-states' because they were similar to independent countries rather than a collective power.

Each city had their own kings, their own army, economy and laws. They traded with one another and often fought wars but were not a single political entitiy.

That is why they are called city-states.