This is a matter of sample bias, basic statistics.
The Nigerians who can legally get into the country are skewed to the well educated or have some money. Legal immigration requires proof of self-sufficiency.
We are not seeing a representative sample.
Its the same reason people in Asia think all Americans are rich - because you need some money to travel there.
The Nigerian immigrant I know has the most elegant written English of anybody I’ve ever met. He completed his PhD and is now a Professor of Computer Science.