Nargott
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A common misconception, not just among the creators of HoMM. It is believed that "supposedly" skeletons consist of rotten bones that crumble from even a light blow. I completely disagree with this.
A true necromancer, wanting to create a truly powerful unit, will reinforce its bones with dark energy to such an extent that it will be impossible to break them at all. Have you seen Ghost Rider? In essence, it's indirectly related.
And what part is actually missing? Only the skin, muscles, and internal organs are missing. Of all this, only the muscles, ligaments, and tendons strengthen the bones themselves; all of this is more than compensated for by good dark enchantment.
As for the heroes, I'm not arguing that the amount of HP here is irrelevant. But all these units are a poor imitation, a couple of mana points wasted on a crowd of weak skeletons, when it's not a problem to spend a couple of thousand and create one - but stronger than an entire army.
From a purely physical point of view, bones make up about 15% of the total body mass, so a bare skeleton is not just fragile, but very fragile, and only insensitivity to pain and enchantments make its HP comparable to that of a living body.