True, some spear become warped because they are kept upright, standing in a corner. I usually 'hang' my spears, point up. You only need a nail and some rope. This prevents them from bending.
Some Central African spears indeed have bend shafts. I think it all depends on the use of the spear. Ceremonial spears will have thicker, carved shafts. The same can be said about the thrusting spears. Strength is a big issue there. What would a warrior be with a spear that breaks in two when he thrusts at an opponent ?
But for hunting, a simple stick/shaft is enough. I don't think an animal will be killed on the spot when hit by a spear. Most of the time it runs off and is tracked by the hunters until it is too weak and can be finished off. When the spear blade is stuck into its back, the shaft is often lost or broken. Why then bother with a nicely straighened or carved shaft ?
Here are some examples of harpoon and/or throwing spears in the collection of the Frankfurt Museum für Völkerkunde (etnological museum) in Germany. The drawings come from a book published in 1985 and are of examples in the Museum's collection.
The last drawing also shows a Mongo spear.
Look at the way these shafts 'twist'.