Multi-threaded CPU optimization is still really really hard. Remember, games are mostly based around input from the player, and the game-logic/simulation responding correctly. Because of this, games are deterministic, meaning: they work best in logical order, mostly on a single game-logic thread. This main thread (combined with the render thread) sends out jobs to the other threads and they are most focused on aesthetic elements that have less dependency on the main game-logic simulation. (Think texture calls, particle calculations, non-gameplay related physics, etc)
This is what make multicore programming -specifically for games- still really difficult to this day. And I bet because they use UE4 -that is not a in-house engine- makes it way more tricky on low-level optimization, because they're dependent on the experience and documentation from Epic.