Why do protease-supplying cells of the stomach and of the pancreas make only precursors of the active proteolytic enzymes?
The stomach and the pancreas make zymogens of the proteases pepsin, chymotrypsin and trypsin and these zymogens are released into the gastric or duodenal lumen for activation. This occurs to prevent the digestion of these organs' (pancreas and stomach) own cells and tissues by the active form of the enzymes. So the production of zymogens is a protective method against the natural effects of the proteolytic enzymes.