What is their reasoning?
However, my experience in the hardware industry leads me to believe it's something about an 'intellectual property' fetish and pearl-clutching when confronted with the idea of 'giving away code for free'.
e.g. when AMD open sourced AMDVLK, they had to replace the proprietary compiler it was depending on with LLVM. That was an easy one. Many projects are much more deeply intertwined with proprietary crap.