No offense meant by this, but I really, really disagree with this. If you want to write about Africa, then you have to go spend time in Africa... no amount of book research and internet browsing is going to give you the information you need to do the job properly.
However, to be fair, Hollywood is equally guilty of shoddy research of the "oh this'll do variety" which is why so many of their products that deal with international issues are so laughably bad.
As someone who has actually travelled in Africa as research for a screenplay, I can tell you for nothing, that getting onto the streets of the country you're writing about is the only way to do it for all kinds of reasons.
Where the lack of "on the ground" research really shows is in dialogue and in cultural differences. So, for instance in Ghana the locals use the word "Abruni" to mean "white man"... and kids shout it at you in the streets as you walk past... and they do the same to visiting Afro Americans, who get really, really upset about it, for obvious reasons.
But of course I'm not saying that is the norm, just that one should have a solid grasp on exactly how they should research their subject matter.
Now, where a lot of screenplays by wannabe writers fail miserably is in a lack of credibility, caused by the writer's lack of understanding of the subject matter they're writing about. I can't tell you the number of "drug" related movie scripts I've read, written by people with no understanding of the drugs subculture, how it works, or even why people take drugs... that's the reason Trainspotting was such a great film... because it's about the closest representation of reality I've ever seen of the Edinburgh heroin scene... and that includes documentaries.
You're right, people should write what they know... and writers should actually get out there and have a life in order to have something to write about. Hemmingway is a brilliant writer because he actually did stuff and then took us into the world's he'd explored. If all you've ever done is watch movies and play video games, then of course you've got nothing to write about.