One thing Gaspode (“Woof?”) alludes to is the fact that there are bows and there are GREAT BOWS.
Any of you who tried to make a bow and arrow in Boy Scouts or such realize that just tying a string to a stick doesn’t produce a devastating weapon.
Most of the bows in history are not very devastating weapons. My suspicion, contrary to Gaspode’s theory, is that arrows would be most effective against small, fast animals such as birds or rabbits. Their greater range and faster flight speed would make them more effective than a spear, but a single shot would still bring down the animal.
Against a mammoth or such, I would rather have myself and twenty companions all flinging spears at it. The greater mass of the spear would make it more effective. The Native Americans and the buffalo not withstanding.
But, I do agree that Australia’s sedentary fauna likely did not require bows.
The Romans largely eschewed bows in favor of javelins. Partly because the development of iron armor made the common bow less effective and partly because the organization of the legion made it possible to re-arm the legionnare during the course of battle.
It was only when they faced the Parthenians who used composite bows from horseback that they were overmatched. A composite bows augements the wood with bone and sinew to increase the stiffness and elasticity. But to use such a bow effectively, especially from horseback, required a lifetime of training.
One of the other Great Bows of history, was the English long bow. The long bow uses nearly six feet of the very springy yew wood to increase the stiffness and elasticity. English yeomen were required to train with them every Sunday to maintain the strength to use them.
To keep a long story from being even longer, my point is that not all bows are as effective as those shown in Henry V at the Battle of Agincourt. Knowing this, it is easier to see why some cultures didn’t need them.