Modern CPUs contain a lot more computing units than cores. For a while, hyperthreading was thought to be a useful way to make use of them. More recently, people have turned to advanced instruction sets like SSE and AVX.
Those things aren’t mutually exclusive. Also firstly, I suspect there’s more “low hanging fruit” in making more software make use of more cores. We’re increasingly getting better languages, tooling and libs for multi threading stuff, and it’s far more in the realm of your average developer than writing SIMD compatible code and making sure your code can pipeline properly.
Threads are of course appropriate to implement high-level concurrency and parallelism. But for fine-grained parallelism, they are unwieldy and have high overhead.
Spreading an algorithm across multiple threads makes it more difficult for an optimizing compiler to find opportunities for SIMD optimization.
Similarly to how modern languages make it easier to safely use threads, runtimes also make it easier to take advantage of SIMD optimizations. For example, recently a SIMD-optimized sorting algorithm was included in OpenJDK. Apart from that, SIMD is way less brittle at runtime than GPUs and other accelerators.