scaling parallelism, game engines nowadays just don't scale very well to the hardware without a lot of faffing about
This is a problem of the minimum spec machine.
On game consoles every machine has the same hardware spec. If you target PS4, you know every PS4 has the same memory and processor configuration and can take advantage of it.
On PC games you have someone running a reasonably-performing system from a decade ago (perhaps an E6200 or Q6600) next to someone with a brand new high-end machine from this year (perhaps an i7-6970). You can have someone with a low-end 2016 system next to a high-end 2016 system. The game needs to be built so that it runs reasonably well on the older machines within the minimum spec. Everything added beyond that is typically non-feature content: more shadows, more reflections, more particles, higher-resolution textures, higher-resolution models, etc.
There is an ever-evolving business decision that needs to be made. Supporting older machines means a bigger potential customer base, but at a cost to design and development work. By default most engines allow support of quite old hardware. You can increase your minimum spec if you want, but it requires work on your end rather than being something built-in to the engines.
