Minato’s Sage Mode is a topic that has been controversial among fans because of its many plot holes. It’s not a bad idea, though.
Naruto is a shonen masterpiece, but there are still many plot holes. One of these plot holes is Minato’s ability, during wartime, to use Sage Mode despite not having been trained in Mount Myoboku. Minato, despite the dangers to his family, doesn’t activate his ability. He then dies shortly thereafter. Minato did not use Sage Mode while fighting Obito and Kurama. However, he knew that Sage Mode would increase his chances to win the battle. Sage Mode is complex and takes years to perfect, so Minato cannot imagine spending too much time on Mount Myoboku for his studies or fighting in the ongoing war.
Minato’s death is tragic. However, he uses Sage Mode flawlessly during the Fourth Great Ninja War. That is something Jiraya cannot do. Despite much confusion about this plot development, there is a plausible reason Minato could use Sage Mode. Minato did not master this ability when he was alive. However, the story is different in Minato’s newly reanimated state.

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What is Minato’s Sage Mode?
Minato can use Senjutsu or Sage Jutsu from Mount Myoboku due to his large chakra reserve. Further enhancing his skills, Minato can enter Sage Mode. Due to his control over the chakra, Minato is able to display the orange pigmentation in his eyes and give the impression of a Sage during the war. Minato, however, admits that his ability to use Sage Jutsu is limited because it takes too long for him to build up enough chakra to enter Sage Mode. He is also unable to maintain the form for too much time.
Why Does Minato Not Use Sage Mode to Defend Obito and Kurama
Minato, as a Hokage husband and father, is caught in a quandary after Minato’s devastating attack on Leaf Village. Minato has to defend the village from the Nine-Tailed Beast. He also has to protect his son and wife, who are direct targets of Obito (a masked man). Even though he’s already at death’s door, he gets separated from Kushina the defenseless Kushina. Minato doesn’t have enough time for his chakra to balance in Sage Mode. Minato’s shoes would rather not drag the fight out too long and instead, they would want to resolve everything as quickly as they can. Minato would’ve defeated Obito but for Hashirama’s cells, which are found within Obito’s body.

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What makes Minato capable of using Perfect Sage Mode in war?
Minato is reanimated as a sacrifice with White Zetsu in the Fourth Great Ninja War. White Zetsu possesses Hashirama’s cells. It immediately grants Minato Vitality and a few Hokage’s abilities. Madara can only use Sage Jutsu after having absorbed the cells and chakra of Hashirama, despite not being able to practice Sage Mode. Minato’s successful Reanimation Jutsu makes him a mere mortal. He transcends every boundary of his body, even his inability to enter Sage Mode efficiently, after which he is no longer a mere mortal.
Minato’s new body has many benefits, but Minato still finds it hard to maintain Sage Mode in such a prolonged battle. His earlier statement is true. Sage Mode is the only way to seriously injure Madara & Obito when they’re in ten-tail Jinchuuriki. Minato appears more like Jiraya than Jiraya in Sage Mode. This is due to the Hashirama cells and reanimation. But it is evident in the battle that Sage Mode isn’t something Minato excels in like Naruto or Hashirama.