The truth is, that most people learn through suffering. They're only capable of reacting to strong stimuli, and suffering is more readily available than strong pleasure. Smart people can learn their life lessons through observing or even reading and imagining. But most of people just would go on and on. Sometimes people are so reactionary, stupid, selfish or short-sighted, that the world needs a great world war or two, to destroy old states, old political regimes, old devotion to religion, old notions in science, old buildings ...you know, everything old. Not that I would like suffering, but it is a lesson to not insist on old things so stubbornly, the more crystallized we like our world, the greater violence will have to come and shatter it. Everything must be able to change, to avoid the unnecessary suffering. Which religions don't understand.

Why doesn't God heal amputees? Well, according to all greater world's religions, when people die, they shed their physical body and later they reincarnate and get a new one. This was also for a long time part of Christianity, Islam and some Jewish sects, along with the classy Buddhism and Hinduism. From the view of eternity, lost limb is just a minor nuisance or a good lesson. Some mystical teachers behind every religion even say that we are parts of God, and so God experiences the world and suffers with us and through us, sort of.

When Christian emperor erased reincarnation from Bible and Islam stopped paying attention to its own reincarnatory tradition, the religion lost its explanatory purpose. Similarly it is with other misinterpretations, that describe karma as inescapable fate. Once shit happens to you, it is a fate and you must not do anything about it. That is bullshit too.

If that mystical interpretation is true, then we are the parts of God with greatest manual dextrity and so it's up to us to learn how to heal amputees and other unnecessary nuisances. Once we develop the knowledge how to, it becomes our full right to eliminate suffering and replace it with much more sophisticated processes of teaching and learning. Certain esoteric source says, that the 'Ancient law of evil' will be replaced by 'Ancient law of divine good-will', or something like that. Which is basically admitting that God is not perfect, still developing just like everyone, and that the planet needs some serious debugging before it will be a comfortable place to live. And people can really help with that process.

Nevermind religion, it got stuck with no answers many centuries ago.