In a world where nature is a divine force, and gods are proven fact, thee is weight to taboos. Animation of the dead works by vague mechanisms, so I'll leve out negative energy and imprisoned spirits - those are case by case issues, and more world flavor than anything.
Animating a corpse defiles the body. It perverts its place in the natural order o things, purely because you (the necromancer) have the hubris to think you know better than reality. You have tainted another person's rest by objectifying them into a useful tool, ignored the implicit desire to stay dead, wholesome and in their respective religion's good graces, and invoked things best left uninvoked.
In a world where evil and good are not subjective statements but tangible, identifiable energy states complete with their own subsets of matter, animating a corpse is rape, mind control, cursing another an hubristic all at once.
If the DM says these are not true, but that animation is still evil, he has much less of a case. But these are the genre assumptions. Bringing back the dead is Wrong (with a capital W), and is overlooked in the case of resurrection ecause that's actually a ritual where you politely ask the divine powers to reinstate someone's life. The divine is suppose to handle such things, mortals are not.
Remember however that killing is only evil if death is seen as undesirable.
This is not necessarily true. One of the core concepts of alignment, which trickles down to everything it touches, is that it's objective. Ending something else's life is bad. The more sentient it is, the worse it is. You can also accomplish good thereby, but that good does not eliminate the bad. Alignment properties are physical properties; acting in accordance with an alignment propagates its specific radiation, tainting matter. That's the universe, not a subjective, mortal accounting.
Dismissing alignment as such is fine, but it changes the question from "why is undeath evil" to "why is undeath evil when removed from te base assumptions of what undeath, evil, and rhetoric are".