There are smaller chocolate brands that do have verifiably ethical chocolate. The big companies do everything they can to crush those chocolatiers specifically. Often they do this by spreading rumors about how ethical chocolate is unverifiable or a lie.
It is tough, because there are so many intermediaries in the business, but it would actually be much easier for the larger corps because they have more resources and better leverage. There are also companies that just lie for profit.
Nestle could, with minimal effort, insist on a functional chain of provenance, and could easily fund all of the enforcement and verification efforts with money that fits into rounding errors on their balance sheet. They won’t, because that helps every chocolate producer avoid supporting slave labor, and eventually it would go away. They want the waters muddy so they can continue to abuse children living in impoverished conditions for profit, while saying “yes, but nobody can verify their chocolate isn’t prepared by child slave laborers.”