Why doesn't the concentration of products or reactants change when a reaction is at dynamic equilibrium?

At dynamic equilibrium in a closed system (for example, in the Haber process to make ammonia), both the forward and backward reactions continue to occur. However, both reactions happen at the same rate. Therefore, as the forward reaction is producing a certain amount of product from reactant per second, the backwards reaction is removing the same amount of product per second in order to make reactant, so overall there is no change.

For example, at equilibrium in the Haber process, the forwards reaction will be making a given amount of ammonia per second, reducing the amount of nitrogen and hydrogen. However, at the same time the backwards reaction is removing exactly the same amount of ammonia and decomposing it to make nitrogen and hydrogen. The amount of ammonia made by the forwards reaction per second is cancelled out by the amount of ammonia decomposed by the backwards reaction per second, so there is no change in concentration.