And that's great for grittier games than D&D. Hit points work perfectly for the heroic fantasy of D&D--they're very simple to keep track of, you have a rough idea of how long you can stay in the fight unless something unexpected happens, there's a lot of room to interpret what a hit actually means, and they give the thrill of being near defeat without also becoming worse at what you do, encouraging going on the attack to drop your opponent before they take you out.