Everyone knows that once you've addressed a problem it goes away forever and no further attention ever has to be paid to it. That's how we fixed racism in the United States back in the 1960s!
It isn't that Nike (or whoever) wants to employ children, what they really want is to not get caught employing children. And they also don't want to pay higher wages, and they don't want to pay supply chain auditors. So if they can get away with paying a rubber-stamp auditor, they can claim they did their due diligence and wash their hands of it.