Something that isn’t obvious when we’re talking about the invention of the wheel: we aren’t actually talking about the round shape thing, we’re actually talking about the invention of the axle which allowed mounting a stationary cart on moving wheels.
It wasn't actually just terrain. It was actually availability of draft animals, climate conditions and actually most importantly... economics.
Wheeled vehicles aren't inherently better in a natural environment unless they're more efficient economically than the alternatives: pack animals, people carrying cargo, boats, etc.
South America didn't have good draft animals and lots of Africa didn't have the proper economic incentives: Sahara had bad surfaces where camels were absolutely better than carts and sub Saharan Africa had climate, terrain, tsetse flies and whatnot that made standard pack animals economically inefficient.
Humans are smart and lazy, they will do the easiest thing that let's them achieve their goals. This sometimes leads them to local maxima. That's why many "obvious" inventions took thousands of years to create (cotton gin, for example).
