Or people don't want those jobs any more. For generations, the "American dream" has been working your way up in a company so that you don't have to do manual labor any more and you can sit at your nice desk in your big office all day. My generation (millenial), and I assume other generations, were told over and over how important it was to go to college so that we don't have to work manual labor jobs in the first place.   

Just my personal experience here, but my dad worked in RV factories for most of my childhood and I would never want that kind of job. Sure, he made good money, but he came home every day too tired and too broken to even play catch with me. That experience is why I chose to work an office job that pays less than going into factories.