because its required real 3D knowledge, it took more than 3 months for experimented 3D artist to work with facerig & made custom & know perfectly the pipeline & process. A real good tutorial, need to take more than 30H if we explain everything. Also even if a tutorial exisitng, he need to be updated, as software are updated and new tools / feature appears, its not something easy here is the raison

also each character are different, a tutorial applied on a human model wil not work with a fish model for example (for the rigging)