Q.Reason out why equilibrium concentrations remain constant?

Chemical equilibrium can be pictured through an everyday analogy: people continuously moving between the first and second floor of a building. If the number of people moving up in a given interval exactly equals the number moving down in that same interval, the population of each floor stays constant even though people never stop moving between floors -- a genuine, ongoing dynamic balance, not a frozen, static one.

Chemical equilibrium works identically. A reversible reaction does not switch off once equilibrium is reached: the forward and the backward reactions both continue to proceed, at exactly matched rates, so the macroscopic (bulk, measurable) concentrations of every species stay fixed even though, at the molecular level, reactant molecules keep converting to product and product molecules keep converting back to reactant, continuously and simultaneously. This is precisely why chemical equilibrium is called a state of dynamic equilibrium rather than a static one -- illustrated by $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, where even once the concentrations of $H_2$, $I_2$ and HI have stopped changing, individual $H_2$ and $I_2$ molecules are still colliding and forming HI just as fast as HI molecules are still decomposing back into $H_2$ and $I_2$. …