Who would control an unmanned aircraft once it heads through Asia, Africa etc? What would occur if the signal was jammed? Would an unmanned aircraft be handed over to different authorities to manage as it passed through their airspace? Who would hand over their asset?
If the technology is here, why can't an aircraft take off from a regional airport in Asia and land in regional airport in Africa without any human input with the pilots just monitoring?
Far too many variables and cost implications at the moment I think.
I don't think computer reliability nor the technological advancements are at the right level yet; the variables in predicting unknown situations and resolving them from the slightest clues on a multitude of inputs remain the domain of skilled humans.