Because, Saliva secretes Salivary juice which helps to makes food as a paste called "Bolus" in our mouth.
Bolus will be same as the outer layer of Chewing gum.
How the soft outer layer of Chewing gum is there, that's like will be there for Bolus.

When we swallow the food, it will be the complex food, which will be in the form of starch. So, the Saliva helps to convert starch into sugar form. Which means, the saliva helps to convert Complex food into simple food (Simple food refers to Sugar).So that, it would be very helpful to push the food through Oesophagus to Stomach in peristalsis or rhythmic movement without any harmful while moving food through Oesophagus to Stomach.

*Saliva helps us enjoy the flavours in our food by helping our taste buds break food down into smaller bits.
*It also helps mash and mix food, so it's easier to swallow and digest.
*Saliva even helps us talk by making it slimy enough for your cheeks , lips and tongue to slip and slide around our mouth to form sounds.