From that criteria, Germany and Austria should be one country.
Parts of Belgium combined with parts of France. Other parts of Belgium combined with The Netherlands.
The entirety of Central America, from Mexico to Panama, can almost be said to be "one country," with the exception of Belize.
Norway and Sweden are quite similar in ethnicity, culture, and language.
Then, of course, in Africa, you have many, many ethnic groups and cultures encased within arbitrarily-drawn political units. In Nigeria, for example, at least three "nations" can be formed based upon the dominant ethnic group and religion.
And this is just a small sampling.
Perhaps, with all the above kept in mind, it all evens out in the end.
Question: Do you believe in peanut butter?