My question is how did the megafauna of sub-Saharan Africa and south Asia survive
until modern times amidst their human presence? North of the Mesoamerican culture
area there must have been millions of square miles where the human population was
little if any denser than in sub-Saharan Africa, and less dense than south Asia, and
the same is true for South America outside the northwestern quadrant with its relatively
advanced cultures. If there are large tracts where elephants, rhino and big cats still
hold out tenuously in the wild in sub-Saharan Africa and south Asia, then the missing
megafauna of the Americas should have survived at least until first European contact.
I will not be satisfied with any indictment of the original inhabitants of the Americans
on this matter until someone can provide a convincing explanation for why they were
so much more effective large animal killers than the sub-Saharan Africans and the South Asians.
Perhaps it has to do with jungle density. While North American forests can be quite thick, I suppose they are not quite as dense as sub-Saharan jungles, or those of South and Southeast Asia or South America. Notice that no one so far has talked about the lack of big animals in the Amazon? Sure, there are no elephants or elk, but perhaps there either hasn't been for some time, or never was. (I don't know)
I do know that new (to man) species as large as deer have recently been found in the Vietnamese jungle, and that in that just three or four human generations ago gorillas were thought to be mythical creatures. The Congo and many other tropical forests are just that vast.
North America is vast, but consider the bounty of tropical nature. Besides more varieties of creatures, there is also a large number of them in a lush environment.
Also there is megafauna and then there is megafauna. Are we talking about mammoths and saber-toothed cats and your La Brea Tar Pits type of animals, or are we just talking hippos and elephants? The mega fauna of the current era is quite shrimpy compared to what once was.
Anyway, why listen to me? What do I know?
I did just fine this interesting article that may help answer the OP:
A 'smoking gun' on Ice Age megafauna extinctions
What do you all think of that?