Creativity. This is the main thing. Maybe a computer can perform infinite calculations per second, but in terms of creativity, it will never be something like a human. As someone said before, think about the BA 777... this is a creative solution. The captain knew what to do without having this experience before... the computer has to be "told" about that situation to give the solution.
So... humans will be in the cockpits, if not forever, for a really long long time...