Ark: Survival Evolved: Low fps becaue of UE4 or due to no optimizations?

AC Unity probably does some fancy stuff with the Crowd-AI or whatever you want to call it. If I remember correctly there are also some very nice clothing simulations going on. If done correctly I am sure you can set these things up in a way that all the idle time of the CPU is used for those things. But that might not be that useful for the majority of games so a general purpose engine can live without it.