I love this question, it is a pure and simple question with a complex answer that will have various interpretations. It is infinitely as complex as the choices a person can make, what an individual does to prepare themselves physically and mentally for, as Eagle sagely put it, the indifference of the world.
But if I had to answer, Why the world is cruel, without being able to refuse the question, I would say it is cruel, because it must be cruel. Without cruelty there is no conflict, there is less of a reason to motivate a person to achieve. Would I go onto do great things because I had all that I desired at my finger tips? No, not likely, that is the path to stagnation and stillness, the unmoving gaze of an empty vessel.
But that is not to say cruelty begets strength, or creativity. Care, love and kindness create these things, empathy, fortitude, charity and humility are the enemies of cruelty who is the harbinger of hate, ignorance, greed and apathy. Yet without cruelty, are strengths would lessen in a light with no contrast.