Hell, I don't think I could do 70-100 reps on the squat with no weight at all let alone light weight.
My guess is that most Bodybuilders would be a crumpled heap on the floor after a set of 50, full body, bodyweight squats.
It's true that squats are a full body exercise, but not if you have a barbell on your back which turns the upper body into a stabilizing mechanism.
Consider why you would squat in nature, either to pick something up off the ground, or to explode off the ground to grab something out of your reach, like a piece of hanging fruit. Neither of those has your hands up by your ears on the descent.
Your arms would be down and back, then brought up and forward powerfully for an explosive move.
That's why sprinters have good upper bodies.