There are quite a few countries who's culture is substantially different from the United States in which bribery is considered standard business practice. If you dont bribe an official in one of those countries, you dont get anything done.

It probably would be more profitable working at the local oil change shop or at walmart, but I doubt those places exist (at least in the way you think of them) in nigeria. Also, if you look at the GDP per capita of the United States and Nigeria you'll notice that the small amount of money made phishing is much more valuable in nigeria than it is in the united states.