Because DayZ isn't done, and unfinished games usually run like crap. I'd wait for it to actually release before you start trying to "fix problems" that are really just part of the fact it's in alpha. There's a huge warning right on the Steam page "WARNING: THIS GAME IS EARLY ACCESS ALPHA. PLEASE DO NOT PURCHASE IT UNLESS YOU WANT TO ACTIVELY SUPPORT DEVELOPMENT OF THE GAME AND ARE PREPARED TO HANDLE WITH SERIOUS ISSUES AND POSSIBLE INTERRUPTIONS OF GAME FUNCTIONING."