@datarama You might wonder "how did the koala get so stupid??? Isn't evolution about survival of the fittest??" and yeah but in this context "fittest" means "most fit for its ecological niche" not "smartest/strongest/whatever".
Intelligence costs energy and eucalyptus doesn't provide energy but it is abundant. (I think there are a couple of other niche advantages of eucalyptus but can't quite remember what they are, probably that it's not really used by other animals or something). Evolution's "solution" (insofar as any of this is goal directed, which it isn't) is to reduce the energy consumption of their brains to the bare minimum... so the smooth brain and lack of higher cognitive structure is actually an evolutionary adaptation. The side benefit, of course, is not naming your fake robot girlfriend "Claudia".
Humans, meanwhile, with their fully-wrinkled brains, have invented multiple kinds of tech that threaten to wipe out most life on Earth, including themselves.