If this question refers to the fact that saliva does not clean as well as water, as in trying to lick your lips clean, and finding that you still feel somewhat sticky, then it must be because saliva is only composed of 99.5% water. The other components of saliva leave a film on your skin when you’ve tried to lick yourself clean…
Saliva contains mucus, for instance, which is composed of mucopolysaccharides and glycoproteins, which are themselves “sugars”, so there you’re trying to remove something while adding some of it. Not too efficient. However, this mucus is essential to forming the slimy lump of masticated food known as the bolus which can then slide smoothly down the esophagus. Plain water would not have this effect.
Electrolytes contained in saliva, such as sodium, chloride, bicarbonate might leave a salty residue. Up to 8 million human cells and 500 million bacterial cells are present in one ml of saliva. Bacterial products (small amino acids, amines and thiols) can be responsible for bad odors and would leave something behind that pure water wouldn’t. 1022
So If you don’t have any water available, you can use saliva to clean up, but it won’t do as good a job!
If the question refers to why saliva wouldn’t clean our teeth completely after eating sweet foods, I agree that would be so practical! The fact is that saliva does get the process started. Various enzymes play a role in breaking down food particles trapped in dental crevices, thus helping to protect against tooth decay. Sorry if that job needs to be finished with dental floss and a toothbrush!
Ptyalin, secreted by the acinar cells of the parotid and submandibular glands begins the digestion of starch in the mouth, during mastication. Lipase has to be in the acidic environment of the stomach to start digesting fats. Antibacterial enzymes such as lysozyme, salivary lactoperoxidase, lactoferrin and ImmunoglobulinA attack bacteria in the mouth.
Saliva is supersaturated with various ions, which keep the Ph of the mouth between 6.2 and 7.4, a Ph range which protects the enamel of the teeth by preventing the dissolution of minerals in the hard dental tissues. By the way, do you know why small wounds in the mouth heal so fast? Because of Epidermal Growth Hormone, EPH, present in saliva! And let’s not forget that saliva moistens the mouth, dissolves the molecules for tasting, and is essential for swallowing!
Saliva isn’t pure water, thank goodness because all of its other components are so essential to saliva’s role in the body. So it doesn’t clean stickiness on the skin or in the mouth as well as a damp cloth or a toothbrush, but that’s not what it’s there for! 1022