Why do you hide your face,
forgetting our woe and our oppression?
For our souls are bowed down to the dust,
our bodies are pressed to the earth.
Why does God let this happen?
The short and simple (but perhaps in some ways unsatisfying) answer is that we do not know: we are not God, we do not have His infinite understanding or the perspective of Eternity.
However, as believers, we have faith in the infinite wisdom and mercy of God and must do the very best we can to be instruments of that mercy for all those who suffer.