There is an easy answers to this: they are just not interesting and they are difficult to write write about.
What do you think why people here write these stories? If you know why, then it is easy to understand why writers don't bother with such kind of things. They are just not interested with such kind of stuff. Almost no one here (even the readers) is interested about that.
Not to mention, most people don't even know what slavery actually is aside from the sterilized versions. At most, you only see slaves being sold or working as maids or something. A lot of people actually don't see what is so bad about slavery.
There is also victim blaming at work here. If people encounter slaves in stories many would react thus: "why don't they fight?" "they become slaves because they are lazy" and other such things. Many people here thinks it is easy to escape such situations ("just fight") so if these slaves remain slaves, then it is their own fault for not doing anything.
If that is not bad enough, many actually want to have slaves (especially sex slaves). It is a fetish. I think that many writers who write OP protagonists are males and have a tendency to fetish-ize these kind of things and objectify women. Why do you think most slaves you encounter in these kind of stories tends to be beautiful women? You rarely encounter male slaves and almost never see children or ugly women. Why is that?
In short, most writers just don't care. They never really think about such kind of things when they write.
now, i completely get what you are saying fam. Slavery in light novels isnt usually depicted in its truest sense. At most its normally a bit of sex slavery, a bit of beating. But not often do you find them being buried alive to accompany their masters or to guard positions of import for all eternity. You dont see them worked to death in salt mines or being used as lab rats, bait, as game to be hunted for sport.
But the absolute fact of the matter is the other side is right too. If they dont fight, it IS 100% their fault. Not for being slaves, but for continuing to be slaves. Its not like they automatically become free or anything, but if a person doesnt try to escape then whose fucking fault is it for staying that way? Theirs. I totally get that they may feel its hopeless, but at the same time they cant just sit with their thumbs up their asses and take being abused or treated like objects without AT LEAST trying to stand up for themselves and plan a way out. You can call it victim blaming all you want. Sometimes it is, however, on the victim to decide whether to keep being a victim or not. Saying otherwise is just bullshit. Someone walks up to you and punches you for no reason and you just let them beat you to a pulp without ever trying to defend yourself. Who decided to let that happen? You. You cant control other people's actions, but you can control yours. Whether you win or lose is besides the point
No one is saying its easy. Or fast. But come on. Its your life. You REALLY dont like something, stand the fuck up and at the very least try to change it. You may not succeed, but at least you can say you did all you could. THEN its not your fault. Maybe you weren't up to the task. Not smart or strong enough. We cant blame you for that. You can, however, be blamed for laying down and taking whatever people throw at you without ever having attempted to fight back at all. Unless you dont have any hands or feet or are mentally handicapped what excuse do you have? None.
People always say to take responsibility for your actions. But you know what, pal? You need to take responsibility for your inaction too.
That said, not all slaves are equal and slaves who were made into slaves for committing crimes for no good reason other than being a dick deserve what they get.
Also yes most authors here probably wouldnt write about that kinda stuff. I probably wouldn't, or would at least gloss over it, but thats just due to the nature of my stories just being about having a good time. I wouldnt call stories like that boring tho. I mean thats just a subjective view, after all. At most it just doesnt fit with what a majority of readers here are looking for.