That's one of the more obvious points of Kubo's shit. Putting anyone in their shoes, if you're fighting a losing battle, you can either run away or go all out. Clearly no one ran away but they sure as fuck didn't go all out. What kind of ridiculous thought is going through the characters' heads as this is happening?
"Oh we're losing but I'm NOT going to do my best because I'm going to conserve all this power that allows me to be at least 10x stronger than I am right now and- Oh wait I'm dead now. OOPS."