Why Don't Miracles Ever Grow New Limbs?

Recently blogger Ergo Sum posted some comments about Mother Teresa. In that post, he note the following observation that he came across on an atheist website:

If there're so many miraculous cures occuring even today, how is it that we never hear of an amputee growing out a new, fully developed, and healthy limb?

This is an important question! Anytime you hear about miracles or someone praying for a cure to an ailment, it is always something that can have any number of things happen to improve the situation, cure the disease, etc. Often the people getting the miracle cure are in third world countries with less high-tech science and medical care available. But not always of course. The point is that the thing prayed for, or the miracle delivered, is always for something internal in the body that people don't perceptually see the cause and effect relationship for (not without instruments, tests, etc.). So it just seems like: I prayed, and a week later I got better. Or the miracle-worker did some ritual, and a week later I felt better.

Well, if miracles can really happen, or if prayers are really effective, then why don't miracles ever grow back an arm or a leg, and why don't people pray for such things to happen? I think that is very telling: both about supposed miracle-workers, and for people who pray for something to happen in their lives.