Answer: Because both reactants and products continue to form.
Explanation: Characteristics of chemical equilibrium:
Equilibrium is reached when the rate of the forward reaction is equal to the rate of the reverse reaction
Chemical equilibrium is attained in a closed system.
The macroscopic properties remains constant like: volume, pressure, energy etc.
The concentration of the reactants and products remain constant.
The reactants and products keep on forming.
Thus when equilibrium is reached, the reaction is still occurring in both directions, but the rates of forward and backward direction are equal so there is no net change in the concentrations of the reactants or products.