I think that one of the reasons for this is the snobbery that is based in our culture and has become more pronounced over the last few decades. Increasingly, youngsters in secondary school have been encouraged to go to university to study academic courses, or if they're not that academic, something more wish-washy like media. Those who want to work with their hands have been increasingly perceived as slightly dim. Why would anyone want to get their hands dirty, when they can earn bucket loads doing one of many non-jobs in the public sector?
So, as a society, the middle classes have come to look down on physical tasks like plumbing, carpentry, engineering, building, etc. The result is that these industries have been deprived of the pick of the best and most able candidates from school. Expertise has not been handed down and has thus been lost. The result is, as we so often notice, that even those in the physical labour markets have a sense of being somewhat second class, since they are looked down on, and they act accordingly.
"Hallo, Daaaarling! I hear young Samantha has got engaged, you must be thrilled"
"Yes"
"And what does her husband-to-be do?"
"He's a sheet-metal worker"
"Oh. Er, well, bye darling, must dash. I'll call sometime"
Ring any bells?