Just cause Minato learned Sage mode doesn't mean he had to use it. For example Kakashi was taught rasengan but doesn't use it. Because it doesn't fit his fighting style.
WHAT?! Are you serious? A offense and defense booster, which Minato sorely needed to even make a scratch on anyone in the big league, with an added sensor ability that he lacked and something he actually perfected (with an asspull, but I digress) and you're telling me Minato didn't use it because it doesn't fit his fighting style? Just... what?!