This is because they won't pay enough to make that kind of work desirable. If you can make the same money with an easier job, why wouldn't you? When I was in high school, lots of my classmates had parents who were manual laborers, and easily supported their families on those (often Union) wages. Not today. Same with flying. When I was starting out, planes rented for $14/hr, and the CFI was $20. Gas was about what it is now (this was during the mid 70's, so OPEC was playing price games). Look at what homebuilt airplanes were supposed to be buildable for. I've got old magazines that boasted of complete aircraft for under $2K. Using old VW power mostly, but whether wood, or tube and fabric (not many all metal), They were commonplace. The time frame for building was often years, both because of it being a hobby, and the suppliers not catering too much to "homebuilders". Now you can get quick build kits, all match hole drilled, and it can still take a long time to get parts and service. We tolerate it in aviation, but chafe at it in other areas of life.