C does have official intrinsics for SIMD on Intel platforms, but the people who are good at writing video codecs don't like to use them because Wintel culture has such bad taste at naming functions (thanks to Hungarian notation) that using them is near-unreadable and it's easier to write everything in asm.
You're wrong. Intrinsics for SIMD are not named by Windows people. They were named by Intel engineers and supported now by all other compilers. Naming is actually in the C/C++ style - __simd_do_something_here() and such
The Intel platform intrinsics have names like `_mm512_4dpwssd_epi32()`. The standardized SIMD intrinsics with `simd` in the name are much newer than any of the code I'm talking about in ffmpeg/x264/dav1d. These are okay, but not being platform-specific of course means you don't get platform-specific features, which you might want when you're doing this level of optimization.
The other problem is compilers (esp. gcc) were traditionally very bad at code generation for them, although these days they're okay at it.
And, a compiler that knows custom SIMD optimizations for every algorithm anyone might ever need, and can recognize when you have coded one of them so it can substitute its SIMD version.
