Because they aren’t.
Dexter, Spock, Data, Castiel, even poor Bishop in Aliens. Their lack of emotion is complete fiction. They say one thing and act with the pure motivation of children who love absolutely.
That’s why they succeed as characters. They recognize that there are human beings who can’t be replaced and behave accordingly. All they have to weigh is how much they need their friends.
“Emotionless” characters engage the audience’s empathy for the primary colors of a child’s perception. Before things are complicated by disappointment and betrayal. True love, true loyalty, true devotion.
The root of the emotionless character’s appeal is how much the reader wants to protect that perfect, childish perception of other people.