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'.
As someone who has worked on a chip design team and had to deal with this stuff... one of the biggest reasons is patents -- software patents. They can't sue you if they can't see what you're doing. There are lots of dumb software patents out there. AMD did get hit with a couple of those, but we were able to beat them because the order of operation didn't exactly match the claims in the patent.
Often it's a mess of third party proprietary dependencies, code owned by subcontractors, etc.
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.
Usually the code is not important, just the specification. Data sheets and explanations of how the hardware acts will do, they can keep their secret code.
I forget where I read this, but I recall that due to patent situation it's practically impossible to build a GPU without infringing on some patent or another. By keeping drivers closed source it's possible to maintain deniability- if ARM released open source GPU drivers it would be trivial for rights-holders to look at the implementation and see what they can sue for. It's not impossible to do this with blob drivers, but harder.
I'm not sure what there is to buy. Just because AMD or Intel managed to do this doesn't mean it's any easier or cheaper for nVidia or Arm. AMD actually has been sued before over this kind of thing. Why not just save yourself the trouble and effort of making an entirely legal and safe open source implementation when you can just keep trucking with closed source?
Nvidia's closed source kernel driver (and others) might be found illegal in court if a Linux rights holder bothered to sue. But unfortunately there is no one interested and rich enough to do it.
