I haven't read the article, but regarding your comment you are treating the departments as if they are completely isolated. I do psychology and it contains many sub-fields such as political psychology, social psychology, biological psychology which basically can be considered as inter-disciplinary fields, bringing these departments together.

I am sure many other subjects have such inter-disciplinary field, sure maths may not go with psychology (although psychologists do need to do statistics) but it goes quite well with physics, chemistry with biology etc. so these departments are not islands on their own isolated from the other fields, each field has much to contribute to other fields.