Why must a chemical reaction be balanced?
Because matter is neither created nor destroyed in a chemical reaction. I might as well ask whether I have to pay my credit card at the end of this month.
For every product particle there must be a corresponding reactant particle. Every chemical reaction that has ever been observed obeys that principle. Burn a given quantity of hydrocarbon in your home or in your motor, and a corresponding, stoichiometric quantity of the oxidation product is released.