But what happened was that, although the part of work that is actually productive has been reduced, the amount of unproductive work has increased to an extraordinary degree; to the point were many, many jobs do not produce anything. — Banno 
One possibility:  The 8 hour day, originally fought for as a ceiling, has become a floor.  Full time work is not less than an 8 hour day, whether 8 hours is too much time, or not.
Another possibility:  Workers, all levels from building cleaners to building designers, turn good jobs into bullshit jobs because they are what the bosses suspect that they are:  lazy, sloppy, malingering, subversive, etc.
A third possibility:  Many organizations have outlived their usefulness and have become bullshit operations.  Everyone who works there is, ipso facto, doing a bullshit job, perhaps in an exemplary manner.
A fourth:  Bullshit jobs are the fulfillment of Cyril Northcote Parkinson's Law:  "Work expands so as to fill the time available for its completion."   [A similar law:  Paper expands to fill the available storage space.] Automation of many functions (like Xerox copiers which make excellent copies with little effort, as opposed to ink printing which requires preparing a master copy, dealing with messy ink, etc.) leaves more empty time during the day.  The empty time is filled with what will inevitably be minimally productive activity.
And more!
I have occupied a few bullshit jobs.  Usually the job could be done in less time than was available.  But... 8 hours, and no less.  I have sometimes fulfilled the boss's suspicion that workers are lazy, incompetent, sloppy, subversive, etc.  And I certainly expected to be paid the same wage, on time, nonetheless.
I have worked for some organizations that had either outlived their usefulness or were never useful in the first place.  Everyone working at these places (usually non-profits) was in earnest, hard working, devoted, and all that.  Unfortunately, the work was futile--like shoveling wet bullshit with a pitchfork.
The 8 hour day exceeds the required time for many jobs. Because of the floor of 8 hours, one might have to fill 4-8 hours (or more) with activities that sort of resemble work-like activity--bullshit, in other words.
When workers are in jobs that are meaningful (where their executive agency actually has a positive effect on the world) they tend to work harder, more creatively, and more efficiently.  A worker who was a total waste in one job might turn out to be a work-leader in a different job.