I guess Atago is obsessed with "protecting" the commander because "protect the commander" is Takao's last wish before she sank. In a way, Atago and Portland are a bit the same since their minds have been messed up ever since their older sister sunk.
Regardless, these two heavy cruisers know they can't take on a fleet carrier of Zuikaku's level. And even if they do, there is no way they want to piss off Enterprise.