He could have had a disease and died in the war.
Or used a seal/sacraficing jutsu to protect his teammates and died.
Or he died vs like loads of high lvl enemies.
Combinations of this all could be the case too.
All kinds of special jutsu users exist.
Take Hidan for example. Without info before hand for the opponent, he could oneshot the strongest of foes ones the curse is active.
Or someone like Deidara with C4.
Hashirama could probably sense C4 though, but still, imagine a huge group of unique high lvl ninjas vs Hashirama, especially if he needs to protect people. could maybe take him out.