There was a story on HN not too long ago where a stereotypical 419 scammer explained why the schemes haven't gotten more sophisticated. Basically, it's a waste of time and resources targeting people who aren't either senile, naive, highly religious, or just plain morons. Using proper grammar and a well-thought-out background story will just bring your scam to the attention of people who might have the ability to investigate or otherwise interfere with it.

Put another way, if you want to steal a million dollars, do it by stealing $100 from 10,000 people. Much safer than stealing $100,000 from 10 people.