BIS have actually published quite a lot of details about why they can't easily use multiple threads, its largely impossible given the complexity of the simulation they are doing. BIS isn't a big studio and they have an incredibly complex game engine so its a real challenge to make it use more threads than it does. It actually runs OK but I do wish it would use more CPUs because I would like to be able to get to a high refresh rate and better GPU utilisation.
You have to bare in mind that in Arma 3 you play on a single map that is 270 Sq KM and you can see up to about 10KM, the graphics are thus impacted by that. They are also going for a look that is more realistic, so the difference between light and dark isn't over done or overbloomed like in some other games. Graphically its much better than its predecessor, but its obviously not the best looking game but I also don't know of a single other game that even has 10KM of continuous map in a multiplayer game let alone one which does it with 60 players and 300 AI.