Advanced AI behaviour can be CPU intensive (which is why ArmA3 server admins have the option of connecting headless clients just to share the load...) and can't really be done on GPU. Realistic ballistics are also very CPU-intensive.
It also probably is not as optimized as it could be as it's barely out of beta
First of all, the reason for the high demand is the UE4 engine. Sandstorm looks awesome even on lower settings, but the engine requires quite alot of cpu power to manage all of it. Maybe UE4 wasn't the best choice for this game, who knows, but contrary to what you're saying i think it looks very good. Despite playing since Alpha i'm still taken away by the far sight across the town on Crossing, or the absolutely stunning view up the streets in Precinct.
And btw, i have an i5 2500k (somewhat overclocked) and an RX580. I play WQHD, surely don't reach 144fps, but it works.