I remember watching some old benchmarks run on my first system, super low framerates etc.
I think the aim is to show what is possible on today's kit, rather than just make something stupidly demanding with the expectation that next-gen hardware will sort it out. Some customers would expect the benchmark to rum somewhat smoothly when using a recently released high-end GPU. Beyond that, there are those that will use this benchmark at events as a tech demo.
The long and short here is that designing it this way appeases their potential customers. They will no doubt release a 4K version of this at a later date when better GPUs are released.