Fifth Element
Legend
Because D&D is, and always has been, incoherent with respect to the abstraction that it uses or does not use with respect to hit point, as the circumstance may be.If hitpoints aren't meat and high HP count doesn't represent superhuman durability, why exactly can you swim in acid, somersault from skyscrappers and all that, and then just shrug off and get back to fight as if nothing happened? No amount of luck, stamina and will to live are gonna help with such feats.
And it's not just hit points. In AD&D it was quite specific that an "attack roll" does not represent a single swing of a sword, but a series of thrusts and parries over the attack round, abstracted into a single die roll. But if you shoot an arrow, that die roll represents a single arrow.
D&D uses hit points to represent different things at the same time. The system is very abstract, and does not stand up to scrutiny. But its advantage is its simplicity.