Because the author wants to concentrate on the interesting parts of the story that his audience wants to play. The Ranch would be boring to play: as a slave, all Monica could choose is between "yes, master," and "remain silent." Monica as a slave would have zero control over anything and the player controlling Monica would have no choices.
DM is using the threat of the ranch as the plot device to motivate Monica to put up with things she would never tolerate if not faced with a fate worse than death. Maybe someday, as a kind of sequel or bonus, he could show what would have (or did) happen to Monica at the Ranch, but that would be a distraction from telling the story and so is best saved for when the story of the game is told.
I would rather have DM spend the time available working on the main story, where Monica has some control of her destiny and the player's choices about her success matter. I believe that, if you think about it for a moment, you'd rather that, as well.