There are two things. Games are by default using more and more CPU power. Even in the long run those are going to cap out to about 4 cores, you still have all the frame call work that DX12, Vulkan, and DX11 on Nvidia, that are throwing on more and more cores. We already see some limited performance benefits from the DX12 stuff, it's the former that is really going to push larger core usage up. As games themselves use more and more CPU power for things like AI, physics, and other CPU heavy stuff, it pushes the other work (like frame calls) to higher thread numbers. So if 1-4 cores are being used specifically for in game codes, then the GPU work would be thrown on cores 5+.
6 Will be a sweet spot for a little while (my guess is '17.5-'20), but it will be a bit of a snowball going forward, we could see circumstances that 8 and 10 core CPU's will have an advantage in the next couple years. Not necessarily enough for people to say "you need an 8c", but enough that people who want the best gaming experience would want to look in that direction.