"...most work songs mainly died out when repetitive physical work was no longer required."

In an interesting drift, Ian, there is still a lot of repetitive physical work being done, even in our great First World societies but work songs are NOT used.

I believe this is due to a range of reasons. To start with today's First World laborers haven't got either the heart or the lungs to sing. When faced with a dull repetitive job they either attack it full throttle and get it done or they feel so oppressed by the prospect of the job that they cannot sing. And there is the peer pressure of the other workers on the site. And the cigarrettes have stolen enough lung capacity that they need it all for the work. Etc.

They have forgotten the value of the work song. There is also a perception that you have to be able to sing like a paid performer. This is largely due to the electronic media (I firmly believe) and will not change. The work song, with a few exceptions, has been relegated to the role of entertainment.