Well the answer here is: other factors. Safe, supervised strength training is great, but construction workers do not have that luxury, but instead heavy stuff to carry in (unhealthy)positions dictated by the task itself rather than your training regimen.

Then there are toxic chemicals on site they are exposed to, which attack lung, skin, bones, muscles. Then there is dust everywhere all the time, wood dust, stone dust, plastic particles, metal particles. All not great for your lungs, skin an eyes. So the strength training alone would be great, and many construction workers do have a lot of muscle mass, but the rest ist just poisoned.