A few reasons, firstly logistics. Africa isn't traversable by vehicle in a lot of places. In Nigeria one of the support vehicles had to go by boat to get round some of the roads. A lot of the roads aren't so much roads as they are dirt tracks, sand dunes or mud baths.
Secondly security, I think there's one road going through Niger, and there is currently a military coup happening there. That road is inhabited by juntas and terrorists. They had a similar problem in Cameroon, an ambassador told them they had "three choices" for routes. One went through Boko Haram territory, one went through rebel forces fighting in the civil war, and the final route was "still very dangerous but only because of bandits and kidnappers". They opted for the latter.
He did get kidnapped for 2 days in the Congo and they had to pay a ransom to the tribe that stole him to get him back. They also got robbed at gunpoint in Angola and had to drive back to Namibia and wait about 3 weeks to get his passports/visas back.
When I said I thought he'd die in the effort I was quite serious, this is probably the most foolhardy thing you could dream of attempting to do.