Why are chemical equilibria called dynamic equilibria?

Because molecules are moving so they change from reactants to products and vice versa.

Explanation:

Hello,

In this case, it is widely known that chemical equilibrium is related with such condition at which the rate of consumption of reactants and formation of products tends to zero so their concentrations remain the same over the time. For instance, if we consider the formation of hydrogen chloride at equilibrium:

We can also see that the reactants, hydrogen and chlorine are consumed to form hydrogen chloride and it also goes back to its reactants in a dynamic equilibrium (because they are constantly changing).

Best regards.