Jumping in late here, but:
To me the answer “why handaxes” is obvious. It seems clear that the handaxe is optimized for some relatively ubiquitous paleolithic purpose (though I guess archaeology is not clear on what that purpse was). And the handaxe itself is a tool that takes considerable time and investment to create and craft well.
If you put that much thought and energy into making a tool, optimized for your own hand, optimized for some purpose(s) that you had to undertake every day, but also with the potential to be all-around useful, you wouldn’t bother with other artefacts or even stone flakes – you’ve got your handaxe! It’s the bush knife of the paleolithic world. And especially in an environment where the quantity of things you carried with you would be limited, you’d want to bundle as much utility in one tool as possible.
If it’s given that you have some recurring task that necessitates carrying a handaxe, as long as the handaxe is suitable for your other tasks, you’ll just defer to using it, rather than having to carry some other (perhaps more specialized or more absolutely efficient) tools plus a handaxe.
