Why do Steel and Ghost type Pokemon have low HP?

I mean, with 2-3 exceptions, all Steel and Ghost types have low HP. They may be big and hefty like Dusknoir or Aggron or Steelix but they have low Hit Points. What were they thinking when they were making it? Couldn't they give them normal HP? Why so low. On average, Steel types have about 70 HP while Ghosts have 60.
They may have so high defenses yet so low HP. It's weird because it's clearly not a coincidence as all steel and ghost types suffer from low HP.

What do you think?

Maybe they figure those types would be too overpowered with more HP? I mean look at Ferrothorn, it's got the typical great Defense as seen with Steel-types, but also pretty good HP -- and it's really good.

Ghosts I think are meant to be more as glass cannons in general. They have low HP but usually have high Speed and/or Special Attack. They can live long using shenanigans like Substitute, but they're really not meant to be walls (except Jellicent, which kinda sucks anyway, imo).

Well yeah but I am not looking at it from a technical way. Just see it from a more realistic way. Like, why do they biologically have low HP. Maybe it's because Ghost types are like 'dead' so they don't have any 'life'? This kind of way.

Oh and Jellicent rocks. Best type ever, great HP and Sp. Def, decent Def and great movepool. Scald, WoW, Toxic, Taunt, Recover etc What else do you want?

She is as good as Blissey. Blissey also has great HP and Sp. Def, decent Sp.Att and great movepool. She can fill multiple roles like Special Sponge, Cleric, Wish Support, Annoyer(T-Wave, Toxic, Attract, Charm), Blocker and everything else. But basically she is both a special wall and cleric most of the times.

Well it's not a biological study - it's a game. You're much better off coming at it from a technical perspective than a faux-biological one. It's because if they had high defenses, high HP, and moderate attack stats, they would be completely OP.

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.

It's honestly to balance out the playing field. If there were Pokemon like Steelix - with an already high defense, mind you - with a base HP of, say, 80 or 70 HP, they would be extremely good. Too good.

For Ghosts? Simple, really. In the beginning, the ghost-types were pretty much meant to be focused in Sp. Attack, or Speed, serving as glass-cannons. They never were meant to really have THAT much HP...

Well, that's just my view of it. Hope it helps. :)

Ghosts having low HP makes sense as already mentioned, because of their somewhat translucent and light bodies logically. For Steels, i dunno.

That's quite an interesting question to ask, probably because they're two of the best types in the game. As mentioned before, it tends to balance out the game. There are quite a lot of bulky Ghosts with low HP (including Cofagrigus, Dusclops and Spiritomb) but there are quite few steel types that we commonly use with such low HP (such Bisharp, Skarmory and Magneton, which have 65, 65 and 50 respectively), So, it does make me wonder whether a defensive role sounds viable on any of these Pokemon.

May I also mention that there are several Bug and Poison types with low HP as well as the two listed.

It's part of an elemental archetype Game Freak is going for:

Jack of all stats - Normal
Glass Cannons - Fire, Ice, Ghost, Dark
Mighty Glaciers - Water, Rock, Ground
Fragile Speedsters - Electric, Flying
Squishy Wizards - Psychic
Clerics/Supporters - Grass, Poison
Stone Walls - Steel
Lightning Bruisers - Fighting, Dragon
Joke Character - Bug

Well Bugs are just bugs so it's logical that they have low HP. Bugs can't much damage. I mean, their body is fragile and easily squished.

Anyway, I think HP is based on the vague idea of life force. You can clearly see how Ghosts have low HP. That's because they are not actually alive. Ghosts are dead thus Ghost type Pokemon have low HP.
Steel and Rock types as well have low HP most of the times because they are inanimate materials so they don't have much life in them.
Bugs have low HP because they are..insects.
Psychic types also have low HP because they rely on their psychic abilities so their body is frail.

Steel types dont have low HP,I mean they might be a little average sometimes but they don't have a low number of HP.If you do think that though,I think the reason is that steel generaly have a High Defense.If you had high defense as well as HP,you would pretty much be making an OP wall.