By that logic why aren’t African and Indian elephants extinct too? Wouldn’t they also be easy prey?
Who knows ? The best reason for the killed-off-by-humans hypothesis is that the timing works.
Possibly the mammoth population was living much closer to the edge, in terms of food supply, than the African and Indian animals, which live in relative abundance. Maybe the mammoths were relatively scarce due to their environment and needed large territories and were thus easier to hunt out. Maybe in Africa and Asia there were lots of other animals to hunt, most of them easier prey than elephants (And this is true; pre firearms in Africa elephant hunting just wasn’t done. “White hunters” were often asked by the natives to thin elephant herds because hunting them was very dangerous using native weapons).
Elephants and pre-humans existed in Africa for millions of years. Humans became lethal to elephants very gradually, giving elephants time to evolve a healthy fear of humans and strategies to avoid and evade them.
Humans arrived in the Americas relatively in a blink of an eye, fully lethal. The American elephant cousins didn’t have time to adapt.
The theory is that African and southern Asian megafauna evolved along with humans, so were adapted to them when they finally became truly effective hunters.
North Eurasia, the Americas, Oz, Madagascar, New Zealand, no such luck.