The answer to this is twofold. On one hand, quality fountain pens were ousted by ballpoints and throwaway cartridge pens over thirty years ago, at least in the USA. Most old-time repairmen retired long ago without training any successors.
On the other, many of the old-time repairmen were fine at doing straightforward tuning and repair as long as spares were available, but never developed the techniques needed to deal with old pens after spare parts ran out and materials became embrittled with age.
The situation with pen repair thus differs from watch repair. The eclipse of the mechanical watch was both shorter and more recent than that of the fountain pen; furthermore, watchmaking and watch collecting had a longer history and a correspondingly developed tradition of restoration. Vintage pen repair, then, is an art that has had to be developed recently, and which is still being developed. The best practitioners are constant experimenters and innovators. As a rule, they have as much work as they can handle, and very few have begun to train apprentices.