We all know that Minato was able to use sage mode, but when the nine-tails attacked the hidden leaf, why didn't Minato go into sage mode and destroy the "Masked Man"
In the 4th great ninja war Minato himself says that he can't use sage mode efficiently. This because it requires a lot of time to gather that much chakra, and he is not adept at using sage mode.
Kishimoto didn't think of it lol, plus if he had used sage mode we wouldn't have the great story we have today so i rather have this great story with some plot holes. just like on star wars why didn't obiwan use force run to save qui gon instead of running normally when he had clearly used it at the beginning of the movie to get away from the droidekas.