But, HP are far too abstract to get that detailed. Undead, particularly incorporeal undead, have no physiology as we would understand it. So, how can their HP be the same as the HP for a human?
Or take various fantastical creatures that simply cannot work, but, we accept them in the name of genre. Any flying monster that's human sized or larger doesn't work. It cannot work. Not without a complete revision of the physics of the game. And that's the point. HP are not part of the "physics engine" of D&D.
Can you do it? Sure. There are a number of games out there that do what you are proposing - typically pretty sim heavy games where the goal of the rules is to produce an accurate simulation of the world. But, again, D&D has almost always sacrificed simulation for speed of play. HP are popular because they work. They're quick and dirty, but, they work.
Sure, when you scratch below the surface, they make about as much sence as a cardboard hammer. We all know that. The trick is, don't scratch. Once you go down the road of trying to make D&D into a simulation game, it changes virtually every aspect of the game. D&D has always been an, to borrow a Forgism, incoherent game. It uses whatever works at the table to judge mechanics. Trying to redefine HP at this point would effectively invalidate every playstyle that doesn't follow your playstyle.
I'd much prefer to leave HP very fuzzy and then let individual DM's futz about hammering it into whatever shape they like.
