Why is creating undead Evil?

[Psyren]

Fire can also spread out of control.

Yet another poor analogy

The key word there is spread - i.e. fire propagates from fire, following conditions such as proximity of fuel, dryness, heat, oxygen etc. Spontaneous undead, as the name suggests, can appear in any environment, making them much less possible to control.

Undead also much, much more dangerous than fire. Forest fires top out at a mere CR 6 - spontaneous undead can be much higher than that individually, and that's not taking into account what happens if you get one that has intelligence, or casts spells, or can make spawn - let alone all three.

The morality of risking one doesn't even come close to that of risking the other.