Here is food for thought. Amputees are not healed because no one in these times believes it can be done. There are stories in the Catholic church of amputees being healed but even I see it as a possible legend. So for example since Jesus could not heal many in his hometown because they didn't believe he could do it, so he couldn't. That is in the bible. Plus he often said to people, "your faith has healed you".
Thus I repeat since we humans have a hard time believing God can cause an arm or leg to reappear on someone who has lost their arm or leg Jesus, who is also God, cannot heal us. It is a requirement that someone has the faith that God can and will do it and those people are almost non-existent. I have added another requirement unintentionally but it is also food for thought. What if we believe He can but do not believe he will. The book of James speaks of this saying if we are double minded we will get no healing. So some people know God can replace an extremity but have a hard time believing he will.
So my answer is: lack of Faith and lack of Trust.
peace and good health to you,
PS Pope John Paul II said sometimes God sees something good in our affliction that we do not yet see and therefore does not answer (with a yes) a prayer to be healed. "Yet" meaning to me we will find out in the next life what our suffering did to help others. So God answers NO but only because it is doing something good in the world and that we will totally agree with this answer and see its merits but probably not in this life. Our suffering that helped others will make our Joy in the next world even greater. This is why we say at the end of our petition, thy will be done, not mine.
PPS I have an agnostic brother who sees coincidences as the answer to all mysterys so you need not respond. Jesus didn't explain the Eucharist in the 6th chapter of John for the reason given at the end of John. Jesus "knew from the beginning who would not believe" it says. No need to explain because they will not believe. I understand this due to my brothers unbelief. He believes in miracles because they are documented he says but it does not prove God did it. this is logical but at the same time shows a stubberness to not believe in God.