But I always thought there was another practical reason - old philosophy is referenced all the time in newer philosophy - you have to read the older stuff to kind of learn the "language" of philosophy with all of the issues that have been covered and are still being covered.
Plus, old philosophy isn't necessarily irrelevant to contemporary ideas - Pre-socratics had theorized about atoms, for example. Or it's not like ethics, epistemology, or phenomenology will suddenly become irrelevant, though theories surrounding them might change a little bit and be refined and lose references to god.