Who cares for hard pieces. Any concert pianist can play the "hard" pieces.
Even if most concert pianists can play the more technically challenging works, I think many of them view certain works as "more trouble than they are worth", in other words, the Alkan Symphony for example may be a cool piece, but that means enormous time and work invested. Same with Godowsky Etudes, etc. So most limit their repertoire to pieces that they feel have something to say, rather than to show how fast their little fingers can move, like Chopin Ballades or Beethoven Sonatas or Debussy Preludes. Of course, then there are those pianists who make their careers playing difficult obscure music, because they can't play Beethoven, Schubert, or Chopin very well - isn't it strange?
In the end it's like Richter said: "Difficult is what you can't do" - - so what's considered "hard" is different for every pianist, I guess...