Because this isn't Fable.
It's not about Stupid Good and Stupid Evil.
It's about actual "role-playing".
So the choices should be in the gray area.
But even then, say there was an old lady that wants some cake.
She's nice, wouldn't hurt a fly.
So why does it need to be a way to be evil towards her?
What reason is there to poison the cake?
Just cause you could?
That's awful storytelling.
Without a proper reason to be "evil" or "bad" the quest does not need to have that solution.
Just like that old guy in Novac.
He gives you a quest to find out who killed his brahmin.
IIRC, there are no evil or bad solutions to it.
It's either help him, help him and sprinkle a little greed on top or leave him alone and don't solve the quest.
So I think that it depends on the role one has chosen.
As I said, my character didn't like The Kings so he never did their storyline.
I could have gotten more exp and money by doing it, but I didn't.
Why?
Cause it didn't fit the role I had in mind.
Same thing here, why do your character want to do the "evil" option?
Why does he/she need an evil option?
Just to be evil for the sake of being evil?
Again, this isn't fable.
So no, every quest does not need to have an evil option just for the sake of having an evil option.