Why the total mass of reactants and of product not change in a chemical reaction?
Matter cannot be created or destroyed is the law of conservation of matter. Like when you have an ice cube it melts into a liquid and when it gets heated it becomes a gas. It may dissapear to the human eye but it is still there. In these changes matter is neither created nor destroyed. Ice, let us say you begin with 20 g of Ice and you leave this out in the Sun, after a while the Ice will absorb heat from the Sun and slowly melts to water. The mass of water you will get is 20g. The amount of water and Ice you will have will be similar.
In this change, the water molecules locked in Ice will absorb energy from the Sun and will free themselves from each other subsequently changes to liquid water. In this process molecules will not be destroyed or created. The number of molecules before and after the change will remain the same.
Another example is oxidation: If an known mass of copper is heated in the presence of oxygen, the copper will oxidise, but if you can accurately measure it the mass of the resulting copper oxide will be the same as the mass of the copper plus the mass of the oxygen that bonded with it. So no mass has been lost.
Because there is a Law of Conservation of Mass.
By observation over thousands of years, mass destroyed here has been created somewhere else in another form. We reflect that in balanced reactions.