Er, Aztecs and Incans definitely had agriculture. They actually had highly complex societies.

What they mostly lacked were antibodies against the numerous diseases brought by the Europeans. Measles, Mumps, Cholera, Tuberculosis, and so on.

They had agriculture, yes, but they hadn't had it long enough to bootstrap an advanced civilisation before the Europeans arrived.

If they had, perhaps Europe would have been conquered by South Americans instead of the other way around.