I think it is a combination of factors, competition only been a small part of it, the percentage of the local population that has the financial ability for air travel is relatively small compared to Europe, or the US. fares ex South america are generally higher then ex US, while wages are significantly lower. Sorta feels like the airlines have figured out that the people who are going to travel are willing to pay for it, and they don't have a need to compete on price. Also inter South America fares(at least southern SA) are high compared to Europe, Asia, or the US, there is also a lack of discount carriers in South America compared to Asia or Europe.

With that said it is sometimes possible to find some good fares to South America, can't really speak to Africa as I rarely travel there. You might wan't to start looking at northern SA first(Bogota, Lima, Quito, and GYE) Spirit Serves Lima. Also Copa can sometimes have some good deals.