I would imagine that the main reason that "major watch brands" don't offer free models is so that they don't put themselves out of business. The same goes for car brands and most, if not all, other commercial products. After sales parts and repairs can be a significant revenue earner, and if free DIY repair solutions were provided then that revenue stream would be lost. Not to mention the probable voiding of any warranties, when the owner opens the watch up, in order to attempt the repair themselves.
Off the top of my head, I can't think of any commercial enterprise that offers free 3D models for parts, unless they have gone out of business (and not bought out by another company) and the plans have been kindly made open-source. I came across an example of this recently, via some obscure youtube video, which was an obsolete, yet well-known, computer hardware manufacturer who had made their blueprints available (circuit diagrams, source code, etc.).
Also, it would require a considerable amount of additional work, by the manufacturer, for all of the individual watch parts to be separately modelled accurately (into a printable 3D file format), and just would not be profitable (nor practical, given issue of accuracy raised by the answer by Tillex72). In addition, and I'm guessing here and will probably be proved to be incorrect, I'd imagine that the pool of watch enthusiasts that have a 3D printer is quite small - so the target audience wouldn't be large enough to generate the demand to make it worthwhile, even if the files were made available for a considerable fee.