Because a liver cell will only grow into other liver cells. You would need stomach cells to replenish stomach cells. This is what "stem cell" research is about. Stem cells are cells from so early in the growth of a living thing that they can grow into any type of cell, depending on where you put it. A stem cell placed in a liver would grow liver cells. A stem cell placed in a stomach would grow stomach cells.
Deep dark is correct. At some point during embryonic development cells lose their ability to become anything and become locked into whatever they are: skin, brain, liver, stomach, etc. stem cells still have the ability to become anything, which is why they hold promise of curing so many diseases.