The intermediate concentration is not changing over time. So this concentration remains exactly the same even though the concentration of reactants and products over time is changing. That's what we mean by a steady state condition. And the only way that the concentration of the intermediate is not changing is if the rate of formation of this intermediate is equal to the rate of dissociation. If these two rates are equal, only then will the concentration of the intermediate be actually the same. So under steady state conditions, we see that the rate of formation of the enzyme substrate complex is equal to the rate of dissociation.