First chariots/war carts were pulled by onagers not horses. So Chariots still appear in the same places as OTL ( Mesopotamia and central Asia ) they just stay more as missile platforms. Onagers are if anything faster than horses, they are just a lot more trouble which is why they were supplanted by them. South America is a bit of a Red Herring as the llama has issues as anything other than a pack animal due to its weak back. The non use of the wheel is probably linked to this as it has been found on toys of the right age.