I suspect it's due to a stage of musical development between learning to play an instrument and being a musician.

It has to be a signicant contributary factor. Musicianship is as much about knowing when not to do all these wonderful tricks that we learn as it is about doing them. The same principle can be applied to ornamentation and dynamics. Restraint and self-awareness usually take longer to master than digital dexterity, particularly in an untrained musician.