simd doesnt make much sense as a standard feature/library for a general purpose language. If you're doing simd its because you're doing something particular for a particular machine and you want to leverage platform specific instructions, so thats why intrinsics (or hell, even externally linked blobs written in asm) is the way to go and C supports that just fine.

But sure, if all youre doing is dot products I guess you can write a standard function that will work on most simd platforms, but who cares, use a linalg library instead.