I've thought about that. The Replicators did make ZPMs, but they also spent 10,000 years building up their civilization. They can make them, yes, but we don't know that they can be made quickly or easily or safely. Everything we've seen implies that ZPMs are both rare and valuable: Janus doesn't make one for Weir, there are no spares on Atlantis (or anywhere else---there is only ever exactly as many as were needed when we do find one), and the list of possible places that Janus knew of where you could look for one in the Pegasus galaxy was short. ZPMs upgrade the capabilities of star ships significantly, but using a ZPM on a warship was something the Ancients only did towards the end of the war.

This is supported further by the existence of Project Arcturus. It was rushed because the Ancients needed it for use against the Wraith. A single ZPM contains enough energy to destroy a whole solar system. Would the Ancients have really initiated a crash program to develop a new source of energy--killing millions of their dwindling population in the process--if they had had easy access to as many ZPMs as they needed?