Because the input bytecode blobs that are passed into 3d APIs are only a gpu-vendor- and render-pipeline-agnostic bytecode format (eg SPIRV or DXBC) - this is not what's actually running on the GPU. There's a second compile step happening inside the driver which compiles into a proprietary "machine code" format, and specialized for a specific pipeline object. Normally that driver internal compile step is quite fast, but that doesn't matter if there's tens of thousands of shader variants.