A good contrast would be Brazil. Most of it's interior is thick(est) jungle and dangerous rivers, etc. Now, while it's not first world, it's certainly doing better than Africa as a whole. A reason for this would be that, as a political unit, it not only has those interiors, but also lots of coastline. Nations that are strictly interior will have it tougher. Still, is that one reason enough to explain the disparity, or is there more at play here? I'm inclined towards more.
Yes, and a fairly good reason too. Interior regions throughout the world are often less developed than coastal and border regions. I've always assumed this is largely because international trade and other economic activity is easier in the peripheral areas than in the interiors.
As to Brazil, the developed parts of that country are broadly situate on the coast, but the political unity allows the economic activity in the coastal regions to carry up the entire economy. (To an extent, of course, Brazil still has broad development issues.) This is untrue of, say, Mali, because why would anyone pass Mauritania or Senegal to get to Mali? The economic development of Senegal based on the ease of trade with its coastal region dosn't extend into Mali.
And yes, I'd say there's much more to it than just this.