> it's widely understood, and scientifically backed up afaik, that strength training is healthy and good for longevity. Yet, if you look at people whose everyday jobs look a lot like functional strength training, eg construction workers, my general impression is that their bodies (age 50+) are in worse condition than the average population...

When it's a work, you're expected to show up and do it consistently every day. So you can't afford alternate days to get adequate rest and recovery time. Your body is gradually wasted away by the job. When it's more of a leisure activity, you can afford just not to do it and rest, when you don't feel well, so the combination of workouts and recovery time can be net-positive, health-wise.