I don't think this is always the case as there are disc authoring companies and disc authors are sometimes also acknowledged in release credits. The guy from Gentle Giant who died not too long ago was one of these, and I think his son picked up the duties for the latest GG re-release (and may therefore be continuing the company). I think a guy from Pineapple Thief also has a similar sideline to help pay the bills. But yes, like any area, some jobs will be handed to less experienced interns, what's the bet that happens in audio engineering as well (which we all know is how SWT got started).
This is a pool thread, so we'd better end this discussion. Pity though, as I have found it interesting.