When I say they cannot write code, I mean that when presented with a simple problem such as toy robot or fizzbuzz which requires them to break down a problem into steps and implement those steps, they are unable to do so.
There doesn't seem to be any particular pattern to this. I've seen it across different companies, different languages, and different seniority levels. I am at a loss as to explain if these people know they are incompetent, don't care, or are just plain desperate for some kind of work to pay the rent. Nonetheless, until we find a better way to weed out the majority of applicants who can't actually write code, this is the best option we have.