Some of this is tradition. The hunting industry in Africa was largely founded on English and German (European, to be more inclusive) practices, with little influence from the US. A totally different safety culture, without many lawyers, etc.
There is a belief among many in Africa that slings are "dangerous", getting caught in the brush, etc. I don't happen to agree, but hey, it is their culture.
If you carry a large bore, heavy rifle (maybe even a double rifle) without a sling for long periods of time, you too will opt for the over the shoulder carry. And yes, in most cases today, the chamber isn't loaded until you are moving in for the shot and the gun is long off the shoulder.
Bill