Ah, sorry, then I carried over an assumption from OpenGL (where GLSL is actually what's packaged, or at least was a couple of years ago when I last looked at it).
In any case, that intermediate representation is probably still easier to compile to hardware-specific code, especially if the hardware has a Direct3D implementation. That's what it's designed for, after all!
Technically true but if you are going to use SPIR-V you might as well jump over to Vulkan. Do you know of any OpenGL games that actually ship SPIR-V? I can imagine it getting used in CAD applications with huge legacy OpenGL code bases but for Games OpenGL+SPIR-V doesn't really have a good use case.
In any case, that intermediate representation is probably still easier to compile to hardware-specific code, especially if the hardware has a Direct3D implementation. That's what it's designed for, after all!