Why are so many scammers from Nigeria?

Its a screen to get the gullible to identify themselves.

Why, given the scam is relatively well known these days, would a scammer still purport from Nigeria or from another West African nation given the association of advance free fraud with the region?

In retrospect the answer to this question is obvious. According to Cormac Herley, principal researcher at Microsoft Research's Machine Learning Department, it's because scammers aren't necessarily interested in seeming believable: They are looking for the most gullible victims they can find, to maximise return on their effort.

The screen ensures that the scammers spend time only with those gullible enough to send money