CP-0 made it abundantly clear that even if Kaido and Big Mom were to both fall, the battle is still not favorable to the Alliances side because of the commanders.
Oda is now sending Zoro and Sanji directly to where the Commanders are. Marco is heavily exhausted, and Chopper only has 10 minutes left until he runs out of Monster point. Zoro and Sanji are the last leg of defense against King and Queen once they arrive on that battlefield. Neither of them can afford to abandon the other one in that fight.
Yamato is the one who is being geared up to go and stall her Father long enough for Luffy to recover and make it to the battlefield. Zoro and Sanji are geared up to stop the Commanders that CP-0 dubbed powerful enough to still tip the battle in favor of the Beast Pirates even if Kaido and Big Mom fall.
Zoro is needed elsewhere, and him abandoning Marco, Chopper, and Sanji would be pretty messed up in the grand scheme of things, when King and Queen have to fall. Unless we are to believe that Sanji is going to deal with King and Queen on his own.
Zoro and Sanji have faith in Luffy and know he will be back. It's the same reason why during Dressrosa, when Luffy needed 10 minutes to recover, Zoro opted to try and stop the Birdcage to the best of his ability, instead of going after Doflamingo, despite there being millions of lives on the line.
This is that time of the arc where Luffy gets removed from the equation and hope starts to dwindle. And then just like in every other instance, when it seems like all is lost, Oda will bring Luffy back in grand fashion just like he always has.