Why do people often treat the idea of a nation and the idea of a culture as synonymous?

There are basic similarities among people within the same national boundaries.

Explanation:

The definition of a nation is a body of people organized on the basis of common language, culture, ethnicity territory, etc. Traditionally common culture is an important element to organized people into a nation. But the nation has other elements too which separate the meaning of nation and culture. And it should be noted here that today a nation is a political entity and the element of common culture remains a part of it.