Many of the film cameras that people 3D print essentially outsource that stuff to off-the-shelf focusing helicoids which are readily available, along with medium or large format lenses which are traditionally mounted in a leaf shutter anyway. So the hard parts (lens, aperture, focusing mechanism, shutter) are not part of the 3D print itself. You're right that 3D printing a serious shutter mechanism for anything other than a pinhole camera isn't really feasible (yet?). It's the light tight box and the film transport that are completely reasonable to print.