> C++ is fast enough

Yeah, it's really not.

There are multiple areas of work, where C++ can be considered a glue language. The high-performance work is then done in explicit SIMD (intrinsics, ISPC, etc.) and/or GPU-targeting languages such as CUDA or Vulkan.

In these areas of work, high performance is part of the design and not something that can be easily added as after-thought.

Also, relying on optimization features such as compiler auto-vectorization is way too finicky - your hot-loop performance may completely break without anyone noticing by someone changing a trivial-looking part of a loop.