It's not just drivers. It's really about ensuring that the folks that maintain the kernel have a way to test the code they maintain. The reasons that we (the kernel maintainers) have for this requirement are varied. But, for me, it's really nice to have at least one open source implementation that can test the kernel code. Without that, the kernel code can bit rot too easily.

Even better is if an open source implementation is in the kernel tree, like in tools/testing/selftests. That makes it even less likely that the kernel code gets broken.

Disclaimer: I work on Linux at Intel, although not on drivers like this Habana one.

One of the points of having the drivers in kernel is that means they kernel can actually run on that hardware. In addition to allowing for testing as others have pointed out, it is also a way to make sure that drivers aren't used to restrict access to the hardware. It ensures the freedom of the platform.

Or they could pull an NVidia, and dedicate a whole in-house kernel team to maintaining an out-of-tree kernel module.

For NVIDIA? More than one.

The Tegra stack uses a totally separate out-of-tree but GPLv2 kernel module (which also works on some dGPU SKUs). It's available at https://nv-tegra.nvidia.com/r/gitweb?p=linux-nvgpu.git;a=sum...

And then there's the partially closed kernel module stack, which is a different code base...