To the chagrin of the watch industry, much of its difficulty finding skilled watchmakers to keep service times low is self-inflicted. Restricting parts restricts the number of watchmakers who have either the ability or desire to make a go of things.
The "Disturbing Reality" of the Reduction in Service Wait Times
Ashton Tracy writing for Quill and Pad:
Understaffed workshops throughout the world struggle to keep up with demand for repair, which in turn causes lead times to swell out of control.
Another big factor not alluded to in the article are the parts restrictions imposed by most watch brands.