Edit: Not to say that most of the times we are talking about small operations, where the PLC programmer is also electrician, plumber, chemical eng., mechanic, etc. ...
We are not only talking about big corps where hyperspecialization is the norm, the bulk of the users are (literally!) jack-of-all-trades, software is only a part, and in many cases a marginal one at that. People who have to do with PLCs are IMO far, far more diverse than the other kind of software developers.