Necromancy is contrived and inefficient. It's more about the aesthetic of being a necromancer than any practicality, no self-respecting mage would devote themselves to necromancy, it's only the edgy, delusional, wack-jobs. Unless the magic system is specifically enabling necromancers for some reason, there is no reason for someone to practice it.
Why bother raise corpses? Golems work better. Even for raising up armies with magic, self-replicating golems are more durable and there's no need for sourcing corpses, there's dirt and rock and wood everywhere. Can have them run smithies too to make 'em out of steel. And liches? Are better as golems. Why puppet a flimsy skeleton with your immortal soul when you could be piloting an armored behemoth of a golem made of solid steel (or, an outer layer of steel with a wooden or hollow core for reduced weight). And as a golem, you could integrate useful tools and weaponry and spells into your own body, far easier that a skeleton or undead flesh bag. Any being aiming to become a pinnacle life form would choose the perfection of steel over the weak and rotting flesh, or naked bone.
Golems, like undead, never sleep nor tire, they are relentless. Damage to the frame means nothing so long as the core is intact. With undead, the core tends to be the head. Exposed, brittle, weak. A golem core tends to be in the chest, where it lays behind layers of steel. Golems are far more durable than the undead, because you can make them solid all the way through, whereas flesh is squishy and easily cut, and bones are relatively thin (in relation to a golem's limbs) and thus more susceptible to blunt damage. Undead are pungent, ugly, disgusting, and a vector of disease. They are a hazard to any living followers you may have, as well as yourself if you are not also undead. And who wants to be undead? Disgusting disgusting disgusting, a living hell, you take all of the vile traits of flesh and then multiply them. And you may suggest that undead being plague bringers is a good thing, weakens the enemy, but waiting is a disadvantage for an army of undead because during that time you aren't increasing your numbers, making you weaker to a direct attack, not to mention that an undead army will continue to decay. A golem army however, increases in numbers while waiting, and each individual is stronger than an undead soldier, on account of having more mass. A golem charge will not be stopped. A golem shield wall will not be broken. Whether a golem army is waiting, attacking, or defending, they have the advantage. And they will outnumber an undead army.
Undead armies start small and build up momentum. A golem army can be built at exponential rates long before anyone has any idea that such an army is being built. By the time people are aware there's an army, they've already stormed the strongholds and capital.
Golems are not limited to evolutionarily viable shapes or sizes. Specialized forces can be designed at a whim for any target one expects to encounter, whereas with undead, you have to find something that fits what you need, and hope you can collect enough. Massive fort-busters, tanks, many-legged many-armed soldiers for crossing any terrain and engaging several enemies at once, with an even more protected core. Small drones for scouting and spying.
Undead are vulnerable to Holy magic, or Light magic, depending on the setting. Golems have no such weaknesses.
There are two possible advantages to undead. One, is using ghosts as spies/assassins. I'd argue golem armies don't need such things given the other numerous advantages they have. Raw force trumps any need for subterfuge. Second is psychological effect. An army of undead will horrify enemy soldiers and reduce morale, and they may struggle to slay their risen comrades. But argue golems can replicate those effects just as easily. By capitalizing on the ability to freely choose the forms of the golems, one can recreate the faces and forms of former living, can make horrifying facsimiles of real creatures that will terrify by virtue of the uncanny valley, and golems can use captured soldiers, encased in living-armor-like golems, with their faces exposed, as hostages, that will affect enemy soldiers morale even more than if they were just dead. Golems could also wear the corpses of the dead easily for fear and disease tactics, and not have to worry about the weight, because of their high mass, and gaining neutralizing plague as a potential advantage of undead, in fact turning it into a point for golems, on account of being able to clean your golems to not spread disease among your friendlies/minions.
Necromancy as a magic serves no purpose besides that as of a tool of a Dark Lord, a conqueror, but a
real Dark Lord leads an army of golems.
And yes, that is the only purpose of Necromancy, the disease makes them too dangerous to the living for mundane tasks. A problem not shared by golems. Golems make excellent assistants, couriers, servants, body guards, and workers, as they can be specialized for any task, without being a source of disgust or disease to the ones they serve. This is useful for after you've conquered the world with your army of golems, as they are still a valuable source of labor. And self sustaining, unlike undead, which will eventually wear out, and without new corpses, be lost forever.
Ultimately, the undead are just lesser golems.
~ Signed, Kaleido, Goddess of Magic, Maker of Constructs
