Yep, it's unfortunately a limitation of, for the most part, DirectX. Games and software can use multiple cores, and threads, to do multiple calculations at the same time. Before dual-core CPUs were a thing, a CPU could only do one thing at a time, because it only had one core and one thread. Nowadays, CPUs can do many more things in parallel.
However, when rendering real-time 3D graphics, in games, the CPU creates the instructions, the draw call, for the GPU to render. Before DX12, draw calls could be created in any thread, but all had to be processed in a queue, one at a time. This meant that only one core/thread worried about sending draw calls to the GPU. And hence the possibility of causing that core to max out at 100%, forcing the GPU to wait. This creates a CPU bottleneck, where the GPU simply has to sit idle to wait until the CPU is done with the draw call.
With DX12, multiple cores can send draw calls to the GPU, all of which will be handled in parallel, thus offloading the CPU load from one core to multiple. But, does that mean games don't use multiple cores? No, games are designed to run on multiple cores/threads. One core can be used purely for draw calls, and others for other calculations that can also take time. If certain calculations can take a while to finish, having another core continue doing other work means parts of the game doesn't have to wait.
Unfortunately, DX12 is a bit of a mixed bag. In some cases it works, in other cases it causes more problems. So I can see why the devs would be reserved to switch over. Essentially, and ironically, the fewer cores your CPU has, the better it can be for gaming. That's because fewer cores with a similar total frequency means each individual core is faster. A larger cache (known as T1/T2/T3 cache) also means it can process more data stored in memory. Fewer cores means fewer things can be done in parallel, but also means each core can process things much faster. But I'm confident DX12 and other graphics APIs will improve, since newer CPUs are getting more and more cores and threads.
TL;DR
It's a computer thing...
!
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