Because the other options are so impressive...?
They are working much harder as a team but they are unbelievably fragile psychologically, are incredibly tactically naive, and have no idea who they are as a team. As soon as Leno gave the goal away there was a 100% chance Chelski was going to get a second and win the game.
They don’t need a complete teardown and rebuild but they need the next best thing. Wait for the summer and get on with it.
Anyone is better than Mustafi. I think they even moved Saka back to LWB or similar for this game. Maitland-Niles has also dropped back. They have options. If they must play Mustafi due to injury or suspension (Sokratis is hurt and possibly Chambers, who should be gone next year too) then he should be on the outside, not in the center. His defense is terrible and he simply backs up to allow the ball to penetrate the middle.
For 80 minutes it looked like they finally "got it". Then the usual collapse happened.
Ozil will be given a full chance to redeem himself because of a relationship with Arteta but he should not be exempt from consequences. Lacazette imo is a poisonous influence on team mentality and should be shipped out. Ceballos being out and Xhaka's self-destruction have further crippled the midfield. Willock and Torreira are mediocre but see a lot of the field.
Pepe must be used to his strengths or sent along. I could imagine him in Laca's place, or permanently on the wing with Auba alone out front.
I think they should consider moving Guendouzi to defense, at least temporarily, along with fellow mop-top Luiz. I know that removes another midfielder but they do have youth and speed there to promote for the balance of the season, then review in the summer when they should acquire another strong CB, ditch some disgruntled players (Laca, Xhaka, maybe Ozil, etc) and look to only acquire high-motor players who are ready to buy-in fully to Arteta.