why do we assume that humans are smart? answer: ego; corollary: we are not
Even if humans are easy to fool (though the fact that we're just now achieving this on a generalized scale after at least 80 years of theorizing seems to contradict this), humans being fooled can result in significant enough impacts where we should still investigate the degree to which they can be tricked, and what methods of discerning are available to us.
We're also approximately the dumbest possible things that could construct the society and technology we have (if we weren't, we would've done it sooner).
I agree that some humility of our own intellect as primates would do us a lot of good in the coming age of AI.
Humblest too.