You're conflating two different things here.
1. The undead are evil, as per the MM. Therefore, creating them and thus bringing evil into the world is an evil act. End of story. (I simply can't grasp how anyone can dispute this)
You know, let us approach this from a different perspective for a second. Talking biology and technology has my mind in a direction.
Creating and bringing something evil into the world is an evil act. End of story. The Monster Manual tells us what is evil.
So, giving birth to an orc or a goblin is an evil act. The Monster Manual tells us that those creatures are evil, and bringing evil into the world is an evil act.
But see, no one is going to agree with this. Everyone is going to tell me that this logic is wrong, because there is no guarantee that that orc or goblin will be evil, they could be good. Why could they be good? The Monster Manual says they will be evil.
Well, those people will likely point out, the Monster Manual also says you can change the alignment of a creature. All creatures in the monster manual have a potential to be good. Even Goblins and Orcs.
Why then can undead not be good? Why can we not apply that same logic that prevents giving birth to an orc from being an evil act to undead?
Because they will then point to the fact that the undead is not alive, that it is made to kill, that it is formed by evil magic and if I just read the MM I'll see that is all true.
Except, being alive or not alive has nothing to do with alignment. And the murder machine, evil magic, and omnicidal spirit find no ground in the spell description. So, why can't we use the monster manual rule to change alignments? Because the lore says they are evil? But none of that lore is necessary for the working of the spell. And if the lore is iron-clad and unchangeable, then the lore also states that the orc baby or goblin baby will grow up to raid, pillage, kill ect. They will also be evil, no changing it, because the lore says so.
But, yet again, no one is going to accept that, they will tell me that if they raised the child differently, the child would be different. After all, that is how things work.
Well, the spell for Animate Dead does not require any of the components that make the zombie or skeleton evil. None of them. So, I can cast the spell in a completely RAW manner and not create a murder machine fueled by hatred for the living. So, if I do so, why am I still bound by the Evil alignment? If the orc baby raised in a non-evil setting is non-evil, then why is the Undead created in a non-evil manner not non-evil?
And, if they are non-evil, if I cast the spell in that manner, then why is casting the spell evil?
We can't even argue that all undead, no matter their creation, are evil. Because we have evidence to the contrary. Argue that all Good Gods hate the undead? I've got evidence to the contrary.
At this point, it seems they are evil because tradition dictates they must be evil. Why is this so? Because despite what the spell that creates them tells us, there is only one way, and that is the way of evil. Creating undead is a simple thing, that has only one possible outcome, you can't change it, you can't approach it differently, it simply is evil because that is what it is.