Ghosts having low HP makes sense. The longevity of an incorporeal entity can't be expected to adhere to the standards of material reality. No, an effective Ghost is the Ghost who focuses its energies into on point on the material plane. Ghosts functioning as sweepers can't be bothered to bolster their defenses because they're too busy ripping faces off. They can only maintain enough steady physical interaction to kill things. The handful of Ghosts who can wall well do so through malice and trickery. Status bombardment, Pain Split, and Spite are their arsenal. Being able to supernaturally cripple an opponent or suck away the enemy's life force obviates the need for massive HP, and, again, a Ghost can only be so physical, and really high defense and mediocre HP is better than high HP and no defense.
It stands to reason that Steel- and Rock-types, being the other non-living elements, would have less HP than other types with more attack and defense to balance this shortcoming, but I still have a very difficult time accepting that a giant snake made of super-dense minerals only has base 40 HP. The giant mineral-snake line could do with a bit more HP (and, for that matter, attack) and it would to only bring a bit more reason to the Pokemon world, but also maybe make them suck less and give Steelix a chance somewhere outside of UU.