Because evolution ultimately reduces entropy in response to stimuli. This process, combined with time, can yield complexity. Countless studies abound ascertaining rudimentary aspects of cognition in even fairly primitive organisms. It's hardly a trait upon which humans hold an exclusive claim - we just evolved the best version of it.
That's another way of asking if we could (theoretically) simulate intelligence. Scientific consensus offers an emphatic 'yes' to this question. We debate the when and how (not the whether) of AGI.
