Stop looking at the PHB for your answer. Look at the Monster Manual.
Skeletons say: "When skeletons encounter living creatures, the necromantic energy that drives them compels them to kill unless they are commanded by their masters to refrain from doing so. They attack without mercy and fight until destroyed, for skeletons possess little sense of self and even less sense of self-preservation. "
Zombies say: "A zombie retains no vestiges of its former self, its mind devoid of thought and imagination. [...] The magic animating a zombie imbues it with evil, so left without purpose, it attacks any living creature it encounters."
Skeletons and zombies are like land mines that will seek out and kill any living creature they find. They are, quite literally, immortal murder machines. Uncontrolled, they serve no purpose but destruction. They are not just animated and given a semblance of life by magic like a golem. They are animated and given a false life by evil. Undead, apparently, are the outcome of injecting magic and evil into a corpse.
Not evil in the moral sense. Nothing so esoteric. No, in the world of D&D, evil is an elemental force that drives the multiverse. Evil itself is capable of physical form, of direct manipulation of living creatures, and even sentience itself. Evil is as real as electromagnetism is for us. Measurable, observable, repeatable, concrete.
And those are the simple undead. Most undead are much, much worse. Mummies are little better than indiscriminate killers themselves; unable to do anything but attack and kill and infect the living with death. Ghouls, ghasts, and wights are all more intelligent and much more devoted to their hunger for living flesh. Allips, Banshee, Shadows, Specters, and Wraiths are tormented spirits, prevented from travelling to the afterlife. Their immortal spirits have quite literally been enslaved to serve evil itself.
And all of them, every last wisp, bone, and tooth among them is bent upon the mindless and total destruction of the living. Every entry for every undead in every edition of the game says that. And all of them unlike life. They do not grow, do not change, do not breed, do not learn, do not create, do not feel. They cannot do anything except destroy life.
And then we come to the undead so potent that they can scheme and plot and know along side any living creature, only they have little to fear from age. The Death Knight, the Lich, and the Vampire. And all they use it for is their own selfish ends, more callous and endless murder, no purpose but self preservation and death. Undead at this level often require murder and the sacrifice of living souls to create. Abomination is too kind a word for it.
Undead are evil. More so than even fiends or corrupt and foul deities. For these creatures can think like living creatures. They feel, they learn, they grow, and they change (at least compared to undead they do). Besides, they're locked away behind the boundaries of their planes, and many are they're pure manifestations of their plane or alignment. Sealed from the rest of the multiverse unless someone should call on them, and directly opposed by angels, archons, and good and most neutral deities. The multiverse knows that such creatures do not belong among mortals. But not undead. Undead are created by taking what is mortal, what is natural, and what is alive from the mortal plane and twisting and corrupting it into an odious tumor of hatred and violence.
So. Who would willingly create such a creature? Create a machine that knows only two things: a) how to kill, and b) that it must kill. And then you seek to control it? What if you should die, or have control taken or lost? What will become of the evil you have manifested and released? What of the pain and disgust and unspeakable horror you cause the living by seeing their fathers, mothers, siblings, cousins, brought forth from where they rested peacefully to serve some petty and insignificant purpose. Explain to them why your needs are so dire that their love must be trodden upon.
Raising undead is monstrous. It's creating a creature of an pure and naive evil, and yoking it for some mundane task.
The lone exceptions to undead being horrific, hateful, malicious, and destructive are rather unique exceptions. Ghosts can be any alignment, but are typically non-violent entirely. They're not so much undead as the incarnation of an immortal spirit that refuses to pass on to the afterlife. They are not created by evil or magic. They're naturally occurring. They arise because they wish to, and little else.
Similarly, Revenants are essentially self-animating as well. They have one purpose: revenge. Still murder. Still hatred of the living. But very focused and limited... and divinely justified. Note that both Revenants and Ghost cannot actually be destroyed. To permanently kill a Revenant and deny it justice, you've got to kill it and then spend a wish spell to make it go away. Ghosts don't even give you that option. The Hollow One from Wildemount -- basically an option to be a Dark Souls protagonist -- is basically a class of Revenant.
The final exception, Baelnorn, were basically a literary device created to make Pool of Radiance more interesting. So rare they're virtually non-existent, and many of those that exist were created by actual divine magic. Baelnorn really are the exceptions proving the rule. Again, the divinely created Baelnorn are basically impossible to destroy as they do not necessarily have a phylactery.
