Just to put your theory to a test, I ran the Witcher 3 and looked at the CPU usage, 33%. big in-house engine and its only utilising one or maybe one and a half of my cores. Its pretty baselss to look at the CPU usage of a game. the fact is UE4 does utilise multi core machines by running threads on different cores, just like any other engine. pointed out already that AC probably uses more CPU because of the simulation stuff in their engine that runs on the CPU, same can be accomplished in UE4.