why would people in the same ethnic group live in different countries
Some people move for their own personal reasons -- usually to find better jobs and a better life for their families.
I'm sure that many of the people of your ethnic group live in one or more other countries. Ask your parents why your ancestors moved here.
Of course, some people came as slaves and had no choice. And we now see many people from other countries who were adopted by Americans because they were orphaned or their parents couldn't afford to take care of them.