Things are always changing in your world.  Your lives are so short there is no continuity.  Just as soon as one of you begins to accomplish a measurable task, he dies, and the next generation undoes all he worked for.  Or, worse, they ignore the lessons he learned and make the same mistakes over again.  I am not afraid that the world will pass me by while I sit here in this prison your shugenja conjured....  I know that in a thousand years, when this spell finally fades away, you humans will still be making the same mistakes you are today, guarding the same wall and fighting the same feuds.