I know that a significant part of Nigerian scammers are indeed working from Nigeria. A scam starts with exchanging messages. Physical contact is not to be expected. It is widely known that scammer often say they are from Nigeria, so often so that the country's name is burned for this purpose. That must be known in Nigeria, at least to scammers.
I would expect that a scammer would pretend to be from a different country. It would make it harder to "prove" that he is indeed from that country. He is in control of the conversation and has many chances to prevent the proof or fake it.
Some of the scammers may be elsewhere and pretend to be in Nigeria only. But "Nigeria" is burned for him also.
So, what prevents a Nigerian scammer from not disclosing the country he works from?