Because it would take to long. Let's say you have a gobstopper and you want to disolve it in acid. You could put it in acid but the acid is only contacting the out side layer. Once the outside layer is dissolved then it can start on the next one, then the next one. But if I smash the gobstopper into dust then put it into acid now the acid is in contact with all of it at the same time. Chewing also mixes saliva through the food which aids in digestion.the stomach also churns the food to help mechanically break it up but it isn't very good compared to chewing so if you didn't chew anything you are making more work for your stomach.