Why can't I grow a new finger?

That's a really interesting question, and it explains why humans are different from things like amphibians. If you cut the tail off a newt, newts can grow back their tails and their limbs. But human cells can't do this, mostly. We do have some cells within our bodies such as in our brain and cells in our skin that do keep on growing. That's why we can heal small wounds. But with something like a finger, there are so many different types of complicated cells, that our stem cells are not capable of making them. They just don't have it left in them to do it. We basically don't have the genes and genetic programme to do it.