For the same reason that you don't continually uproot a flower in your garden and move it from place to place. It will not bloom as it should and will not get the nourishment it needs and will not be able to contribute to the overall welfare of the garden. I know this is overly simplistic, but it does serve as a small illustration. Notice I said nothing about someone's salvation.
Sort of like the old bumper sticker: bloom where you are planted?