Lookup tables are the big thing that break CPU and GPU programs since at relatively small sizes you'll hit the memory wall. A lot of algorithms like media codecs (audio, video) and pretrained neural nets are trivial to design with HLS and have a major performance advantage over using a CPU or GPU.

Add in the fact that CPUs and GPUs are expensive as fuck, in short supply, demand high power, while FPGAs are pretty cheap and efficient since they don't do much you're going to find a lot of designs that call for them over a beefy and expensive chip.

That said, arm and risc V cores are dirt cheap. If your algorithm isn't hitting the memory wall it probably isn't going to be faster via HLS.