Amdhal's law in a nutshell.Having all the multi-threaded libraries in the world do you little good if they account for only 20% of total CPU time not because you aren't using them but because they represent only that much of the application's total processing workload.
Also guys, threading does not work the way you think it does. Games have been using multiple threads for multiple tasks for literally DECADES now; even DOS games were multithreaded to an extent. The primary reason why we're only seeing games have multiple threads doing significant work now is the graphical APIs support it.
In ye dark ages (prior to DX11), the primary rendering thread was the only thread that could render to the GPU. As a result, a good 60% of your total processing was bound to just one thread. The only other thread that would typically perform a lot of work was the main game executive. That's how you ended up with games that did a good 95% of their work in just two threads.
DX11 gave a limited ability to spawn multiple worker threads; you saw early Frostbite titles use this, and you ended up with two heavy threads and a bunch of lighter ones. A quad was still sufficient as just two threads were still doing the majority of the total workload.
DX12/Vulkan allows much more defined control of the GPU pipeline, so multi-threaded GPU rendering is now possible. So developers will now happily spawn a dozen or so GPU render threads, each responsible for one particular part of the GPU pipeline. While this gives a large amount of control of the rendering process, on CPUs with fewer cores you run into scheduling difficulties keeping the GPU fed (as it now wants to do multiple things at once instead of being a single-stage pipeline). That's one reason why quads are starting to have issues at the highest presets, as there aren't enough cores to process all those GPU threads, even if the total processing workload is identical to what it was several years ago.
In regards to RAM usage, the reason requirements on the PC side are going up so fast is games are now native 64-bit. There's no more 2GB or 4GB requirement on RAM usage. Which means, if there ever is a RAM leak (which as we all know will NEVER occur in a released product ), it will happily gobble several TB worth of RAM, instead of stopping at the 2GB or 4GB barrier.