There is a big difference between responding to every single piece of writing and responding to most pieces of writing. It's idealistic to believe that we as teachers can respond to every single piece of writing with the same amount of attention and care. Teachers must be realistic, not idealistic. Realism allows room for idealism, but idealism doesn't allow much room for realism.
Teachers have lives too.
They have families and interests outside the academic environment. They also need a balance between their professional, and their personal time.