I have often wondered about that, Stan. For one thing, Africans seem resistant to heat - one often sees businessmen in three-piece wool suits in hot weather which has me wilting (I couldn't survive in your part of the world). I think it is also a fashion statement - one is more likely to see a poor or rural African in shorts than an urban middle class person, who is more attuned to social expectations. And in the case of someone employed by a safari company, the boss might require his guys to wear a uniform.
In my two trips to africa, most of the black men or coveralls actually. This was in namibia and south africa. I assume other fashions are common in other countries. Africa is not very homogeneous.
I used to wear shorts a lot in Southern Arizona when hiking off trail up sky islands. Bare legs did not hang up in the cat claw quite so badly as pants. Either way, I got scratched up pretty good, but it was not insufferable. Tearing up pants every hike would have been expensive.
Cactus, thorns really did not care if you are bare legged or wearing pants. You are going to get stabbed.