Because in every chemical reaction MASS is CONSERVED. This has been the routine and consistent outcome of every chemical reaction EVER performed.
What do I mean by "
Well, you start off with 10 g of reactants from all sources; you can get AT MOST 10 g of products. MASS is thus conserved in the chemical reaction: garbage in from all sources, MUST equal garbage out. So, of course, when we write a chemical reaction, for every REACTANT particle (which has a precise and definite mass), there must be a corresponding PRODUCT particle (with a precise and definite mass). If there is not, then someone (most likely you) has made a mistake.
Balancing the equation shows us how matter is chemically transformed in the chemical reaction. Every chemical equation must demonstrate this principle of conservation of mass.