Weight lifters can go beyond 50+ lbs, safely, only if the weight is basically in barbell (or dumbbell, or whatever) format. A large 100lb box isn't being lifted by any smart weightlifter alone.
OTOH 73kg (160lb) full beer kegs still get a fair bit of one-man manhandling, and to be a proper professional scaffolder you have to be able to lift a full-length scaffolding pole (just under 30kg, apparently?) from flat off the ground to straight up above your head, partly one-handed in one smooth movement, several times a day routinely.