It is a good question and i will try to give multiple answers to "why corsairs and slavery don't mix"
1. the oorp one: Slavery is closely connected to the Outcasts - ad at least back in the day the sairs were the anti-mirror to them
2. the practical one: if you are not using slaves for yourselves, who are you going to sell them to?
Factions i remember engaged in slavery/using slaves/slave trading are the outcasts( => the sairs don't want to support their economy for sure!!!),
the rouges (=> aka, the lickspittle pushers of the outcasts, we don't want them to have better relations, those suckers better buy artifacts),
the lane hackers (=> cardi junkies and outcast fanboys as well,
the Mollys ( why freedom dudes do it /did it is out of my mental scope, but sairs don't support their war enemies)
the hessians (see above)
the Zoners( as i know of no zoner faction that uses slaves they obviously sell them to one of our enemies)
the Junkers( same as zoners)
===> if you trade in slaves, you directly support our friggin enemies!
3. the foodsie one: Slaves need to eat, drink, breathe.
The sairs play is that while no one needs to starve there is no surplus in supplies either - and corsair population is limited by supply. Why would a gringo hating corsair (who could do more honorful duties than slaving) feed slaves if he could feed poor corsair workers, too( see below!)
4.the "moral" one: Slavery is an outcast trademark, the corsairs hate them with a passion, as such i suspect that corsair society hates on "typical " outcast behaviour, marking it as "honorless", cowardly, wrong, whatever.
My personal take always was this: a corsair has a firm "stick of honor" up his rear, which makes him believe he is morally superior to the non-corsair lowlife(aka everyone else) - this is why the sairs mix better with kusarian factions that have more traditions and codes of honor than other factions....
In any case: My Corsair char captures let's say a hessian and has to decide what to do with him - he has no valuable info being a low grunt...he could sell him to slavery and earn some cash - but for him that would lower his honor as he would do activities frowned upon by society(as explained above) - in all his generosity, he let's his prisoner keep his honor (and preserves his own as well) by executing the poor chap, in the firm believe he is "helping" him and being "merciful".
5. Support local labour: in a society that revels around strenght and warfare, there are losers ad probably a lot of them - i believe that the corsairs have quite a lot of war veterans and poor folks that are not/ no longer able to keep their good standing with the rest of the society, i don't believe there is a good healthcare or social security system in place for a society that just a decade ago had starvation problems and would rather prefer to kill off the weak and unproductive instead of "wasting " resources on them. Imagine a horde of starved cripples on the streets of Europa - now imagine a rich sair landlord who owns a goldmine offers them a cup of rice and a bottle of water a day....if they work his mine....you know where i am going?
I am sure the corsair society preys on the weak and exploits them to literal death, with no or only very little repercussion, thus creating "de facto slaves" who are none on the paper - and don't shorten the honor of the owner, but instead making him a benevolent giver for those poor souls that were threatened by death
6. The PR-stunt one: slavery is frowned upon by about everyone save slave users.Corsairs are frowned upon by everyone. The rp (at least the benitez one)
was to set captured slaves free on freeports to hopefully spread the word about vile outcasts and honorable corsair saviours, thus maybe maybe maybe giving us an advantage of being a bit more "civilized " than others and opening up some more opportunities.
===> short version: Slaves support our enemies, we have better things to do, say no to slavery!