There are often strong disincentives, as well. Nike, for example, got themselves into trouble when parts of their supply chain became public. Many more companies regard their unique supply chain as a competitive advantage. So, even if you should take up the time, effort, and expense to research your own supply chain, there's a high likelihood that your upstream vendors will have no interest in sharing their suppliers with you.
There are exceptions to this. Reportedly Apple has excellent supply chain visibility, but they've achieved it at great effort.
(I'm by no means an expert, but I used to work for sourcemap.com, an MIT startup based around the idea of understanding supply chains end-to-end.)