Because, empirically, we have numerous unique and differentiable qualities, obviously. Plenty of time goes into understanding this, we have a young but rigorous field of neuroscience and cognitive science.
Unless you mean "fundamentally unique" in some way that would persist - like "nothing could ever do what humans do".
> People constantly make comments like "well it's just trying a bunch of stuff until something works" and it seems that they do not pause for a moment to consider whether or not that also applies to humans.
I frankly doubt it applies to either system.
I'm a functionalist so I obviously believe that everything a human brain does is physical and could be replicated using some other material that can exhibit the necessary functions. But that does not mean that I have to think that the appearance of intelligence always is intelligence, or that an LLM/ Agent is doing what humans do.