He says he's bad at it for some reason.
Summary:
- Minato's Sage Mode is a plot point in Naruto that confuses many.
- While he clearly mastered the technique, he probably never actually used it.
- Fans wonder why he didn't use it against Obito.
In Naruto, Sage Mode is a special kind of ability that allows the user to tap into the natural energy of the world. Natural chakra gives access to a variety of abilities, but Sage Mode is notoriously hard to master.
Outside of Naruto himself, perhaps the most famous user of Sage Mode is Jiraiya — who also taught it to Minato Namikaze, the Fourth Hokage. However, Minato's Sage Mode has been a point of discussion in the fandom for a while — it's quite confusing.
There are a lot of speculations regarding it — notably, Minato himself states that his Sage Mode is weak, and fans can't figure out why.
Chakra Requirements
An important point about Sage Mode is that it requires a tremendous amount of chakra to master. It's easy to see why someone like Naruto or Jiraiya would have the ability to do so, but Minato wasn't stated to have a lot of chakra.
Granted, his chakra reserves are probably still above average, considering he invented Rasengan and mastered Hiraishin — his iconic teleportation technique, which, in the present timeline, requires three people to work properly.
Minato still says he's bad at Sage Mode, and that he can't keep it up for a long time. Whether it's because he never bothered to learn it past a certain level or because he's not someone who's suited for the ability is never elaborated on.
Minato Probably Just Never Got to Use It
Minato not being good at Sage Mode gets even more confusing considering he's not even self-taught: he studied under the Frog Sages, and is presumably well-versed in the nature of the technique.
In fact, Minato being able to enter Sage Mode by himself should mean he's actually better at the technique than Jiraiya. The latter can only enter it by merging with Toad Sages, which should mean he's worse.
Except, Minato never really showed any special abilities outside of those normally granted by the Sage Mode, hinting at the lack of practice. He even states himself that he never used it in a real fight. That said, he finishes his fights in mere seconds, so he doesn't need to use it.
Fans Feel This Is a Plot Hole
Minato being able to use Perfect Sage Mode — that means he doesn't look like a frog when he enters it — is considered by many a deus ex machina moment during the Fourth Great Ninja War.
After all, that ability came out of nowhere, and if he had Sage Mode all the time, he would've beaten Obito during the Nine-Tails attack.