Wheat looks like grass if you aren't familiar with it.

ditto potato plants - only the roots are what you want, and as they're invisible, if you don't know what a potato plant looks like, you'll have no idea you're looking at food, as OP says.

I could go on for a while, but you get the gist.

Not stupid, they just don't know what a lot of food looks like before it's been processed into food. Not their fault, just a fact.