My first job was at 13 working with a friend's father paving. OSHA? Child labor laws? No one cared. Payed me cash to walk next to the paving machine and throw shovels of asphalt and tamp edges. What did I learn during this summer job? It sucked! Everyone there by 45 looked like they were 65. Most spent every night drinking at the VFW. No one liked what they were doing and they were there because they had no other options. It taught me one of the most important lessons in my life - I didn't want to live my life that way. As far as ship building and manual labor jobs, many did it because they had to do it, not because they wanted to do it, and if given an option to not do it, they'll take that option, especially if it's less physically taxing and pays better.
The fact is other countries have large populations with no other choices than this type of labor and that's quite an incentive to continue doing somettting trades for hing you don't really like doing. Heck, there's an entire population of people in the US, who weren't born in the US, who tend to do jobs no one else wants because they too don't have any other options.