Odo Oya or River Niger; Simple Questions?

When I arrived in the United States thirty years ago, I couldn’t call the US a new land.
It might be new to me, but it was not new to those who were born there.
But if the history of the United States were written by me, I would call the United States, “new land.”
That has been the experience of people in Africa.

The lands of their ancestors were declared “new land,” and open to anybody to take and possess.
In this strategy of dispossession and reallocation, history was rewritten,
People simply pretended that there was no existence before the arrival of the Europeans.
The colonial authorities established educational institutions to psychologically recondition the youths of the lands they colonized.

Let me use a personal experience.
The biggest river in the land where I was born is called Odo Oya by the Oodua (Yoruba) people living there.
The colonizers simply ignored the old name and renamed it River Niger.
Then they opened schools in which they taught the children of the colonized people that the “River Niger was discovered by Mungo Park.”
As a child, I was required to memorize this lie to be able to pass my exams.
How did all these things happen?
Why didn’t any of my teachers tell me it was just a lie?
Why?
Are these questions too complex to understand?
Could someone explain to me why my teachers taught me these lies?
What other lies did they ask me to memorize?