I have a suspicion that a lot of these supply chain compromises are done by the security researchers at security vendors, selling software to protect the software supply chain. Spreading fear to create demand for their products.
Like in the good ole days of Windows 98 and antivirus era, a lot of advanced virus techniques in the wild came from the people who used to work for AV companies
That would be extremely surprising, given the number and severity of federal crimes involved.
(I also dare say: many of these attackers demonstrate a better in-depth understanding of packaging ecosystems than supply chain security vendors do.)