As someone who spent a ton of time in this space(working with most mobile GPU vendors from Android E to L) you're talking about things just at the surface.

The way the drivers + backing architecture are built contains tons of IP and things that they would very much not want their competitors seeing.

Our team built our reputation and relationship on keeping that data separate and confidential despite working with just about every vendor out there.

Adding the double blind optimizations leading to absurdly complex drivers that tries to guess how user code tries to guess driver logic... maybe vulkan will help open source GPU drivers in the end.