devtrackers.gg
There's no magic option "use all my cores" the most game code is single threaded
^^ This.
Also, to clarify: -USEALLAVAILABLECORES isn't a valid command-line / launch option. That's one of Unreal Engine 4's built-in command-line arguments. Unturned is made in Unity, not Unreal Engine 4.
Players looking to configure their launch options should refer to the official documentation that we link to from in-game, or use our support site.