Because we do not stop to love our neighbor after they are dead.
But, if as you say, Purgatory is a necessary and GOOD thing - joyful was how it was actually portrayed - then what purpose would there be in accelerating the process? Why would, how could, someone circumvent all that needs to happen in this place of cleansing? They "know" they're Heaven bound, right? Why should prayers, masses, alms and good works be given to expedite this process? It has nothing to do with loving our dead neighbors.