Why is protein digesting enzymes in the stomach released as inactive enzymes or zymogens?
Enzymes are known to accompany a chemical reaction by helping in its process.
It is necessary that the enzymes be in their inactive form to avoid it's reaction with other cells/tissues.
For example,
Trypsinogen is the inactive form of trypsin acting in the intestine (produced by the pancreas). It is converted into Trypsin with the help of enzyme Enterokinase which is produced in the Succus entericus.
Trypsin is known to digest proteins converting bipeptides and polypeptides into amino acids. However, apart from the protein content in the food present in the GI tract, there are also proteins in the cells of the smooth muscles which forms the alimentary canal. Cells have their plasma membrane made up of phospholipids and proteins. If trypsin is present in its active form all the time (i.e. even in the absence of food in the GI tract) the enzyme can digest the protein of the smooth muscle cells, thus distorting its structure and function.
Therefore, it is necessary for the enzyme to be present in its inactive form.