Your question is a bit like asking, “If the Mass has infinite value, why should I have to go to more than one Mass in my lifetime?”
The Mass does have infinite value, but we are finite human beings who generally receive the saving graces of the Mass within the context of time. Because we are creatures subject to the limitations of change, the Mass’s infinite value is applied to our souls over time.
Now, we do not know exactly how this works in the next life. It may be that only one Mass suffices for the release of a particular poor soul in purgatory. But because we, in this life, must offer our prayers within the context of time, so it is good for us to arrange to have more than one Mass offered for a deceased person whom we love. If that person is no longer in need of the graces of the Mass for the release of his soul from purgatory, God may apply those graces gained to another poor soul.
