It's because even brands that go out of their way trying to exclude farms that use slave labor can't guarantee that every farm they work with is slave free, because a lot of the time the slaves themselves lie about their info out of fear of losing the income or provisions, so it takes A LOT relative to how much checking can be afforded for each individual farm to 100% guarantee "these guys aren't using slaves"
When it comes to chocolate the slaves in question are generally children who are made to work by their parents. The parents teach their kids to lie about their age.