There are some truly abysmal people in technology. I work in a multidisciplinary environment and deal with not only coders but people from other sci/tech areas and there's also some pretty bizarre, completely incompetent, personalities in other fields as well.

They seem to exist through some kind of combination of manifesting some kind of acute highly-sensitive HR problems (health, special class, etc.), bouncing around from unsuspecting job to job to keep the paychecks coming in, and simply being walking Dunning–Kruger manifestations. Eventually they accumulate enough time in enough jobs that it gets easier and easier to get new jobs due to being experienced and some of them will end up in senior positions where they no longer have to code (or perform in the core of their technical area).

They can come in from all sorts of places, often have stunning educational credentials and backgrounds, often have great looking resumes.