From what i've pieced together from reading internet forums, a lot of this came up as a result of the HIT nonsense that was once in vogue. The idea of doing one hard high intensity set and going home promulgated by Arthur Jones or whatever it was, was intended to get guys in and out of gyms quickly since they were mainly in the gym business.
You don't want ppl sitting there in the squat racking doing 8 sets of 5 or something because it inconveniences other customers. As far as HIT goes, It didn't really work though because ppl didn't make good gains and it's kinda well known now that naturals need a lot more volume to make progress. Anyway just food for thought, a lot of this stuff is just conjecture and I doubt it matters in the end. Maybe the explanation i've given is bs too, who knows.