Because they dont have to worry about his pace getting behind them. When he was on we could afford not to sit so deep because of his high work rate. He's gonna be slagged off for his performance today, but he really did what was required.
The one move that confuses me is cahill over Luiz for this match. Luiz being able to spot the long pass would seem great for at least keeping some threat on them if they play their possession game.
Even better the high pressing stops Ramsey and Arteta. Throw them off their game and you have essentially nullified Ozil. This makes Giroud a nonfactor.
I'm not convinced Oscar has completely gotten over his injury. And Mata on the wings would be suicide with them bombing up the flanks against our fullbacks.