Probably because it was never designed to handle thousands of AI zombies in-game.
Its what is essentially a good engine that just gets knackered because people are desperate to play with terribly animated and useless zombies for some reason.
that and that you tend to have to install another 10 mods just to play on a particular server, to run around hiding and collecting guns.
Considering Day Z is one of the main reasons Arma 2 started topping the Steam charts and the success of Day Z spurred other modders to really up their game in regards to content. Naively I thought Arma 3 would really focus down on multiplayer performance/mods and AI.
I keep checking out the odd stream to see if much has changed but it's still the exact same issues that plagued Arma 2. Don't get me wrong, I had probably some of my best gaming moments through Arma 2 but I also saw the limitations of the engine.