Ever try to rebuild a embedded computer from RPM or Debian sources? Good luck. While the distribution folks do provide source code (it’s open source, after all), rebuilding from sources is definitely not a use case that they typically support. In most cases, after rebuilding from source packages, the resulting system wouldn’t be considered the same distribution and therefore is unlikely to be supported by the vendor solutions.
While this might be acceptable for enterprise or solutions Linux environments, it’s not acceptable for embedded product companies. Typical embedded products are in the field for years, and they often need to be maintained and updated after shipping. It’s critical to keep a detailed manifest of all of the software included in every shipping build of an embedded product, as well as the tools and scripts that were used to build it. A number of modern Linux build engines provide this capability today, so make sure to select one that does.