Of course. But if you can't publish games with bad things in them because those bad things happened in the real world too, you're restricting the tools we have available for creativity.
Nobody is stopping you from publishing whatever you like. If the market does not support your product - for whatever reason - it's not the market's problem, it's yours. If you receive lots of criticism of your product, one reason might be that lots of people have taken offense at it.
Yes, lots of bad things have happened in the real world. Should gaming products include torture? Racial persecution? The systematic oppression of women? Mutilations? The marriage of twelve-year old girls to fifty-year old men?
If the answer is
no, what makes slavery different?
If the answer is
yes, what is the target audience for your product?
It seems to me that this nebulous "we" to which you refer is really about big publishers like WotC and Paizo not including slavery in their products. No-one is stopping you from including it in your games. No-one is stopping you from incorporating material from third-party publishers which describe it. No-one is stopping you from publishing a product which includes it.
How is it even remotely reasonable to expect the industry leader(s) to publish material which defies social and moral expectations, is found offensive by a great many people, will alienate parents, which will draw negative attention to their brand, and will impact their bottom line?
Because history? Do your games always comport with plausible social-historical models in all other areas? Have you eliminated nonhuman sapients, spells, dragons, flying castles and anachronistic weapons and armor?
If not, why not? Why is slavery different?