One might also say - they're not smart enough to realize that they are smarter. [But this is another PoV].
Engineers don't have any intrinsic smartness advantage over non-engineers. We're (much) better at some things and (much) worse at others.
I guess I should have added some smiles to my previous comments :)
Tbh I would expect higher intelligence among engineers vs the general public if only due to selection effects. Smart kids tend to be more pushed towards professions under that label and are better able to meet the challenges associated with getting a job there such as earning a degree or learning enough to be useful.