Most young guys need to promote themselves for the second and third jobs, etc. Like it or not, a longer resume reads better than a shorter one in most cases. Thus, for a few years, most young guys will list courses they worked on for their mentors, hopefully (but not always) noting they were apprentices.
Most heads of firms prefer the official credit to go just to them. And, in reality, while an staffer may routed the course, another do the field work, others do the final plans, the head guy makes it possible, sets the tone and style (I was always amazed, despite having some talented associates with a few years experience at how much they missed and needed some critique and change by me). Of course, when things go wrong, or reviews are negative, the boss man has to take those, too, not the associates.
Now, like TD, most of us are glad to acknowledge at least the lead associate architect on a project. In truth, guys at the end of their career, if they haven't done it yet, ought to write a small narrative on each project just for posterity sake, including who the main associate was, owner's rep., first super, etc. There are different reasons for giving credit - marketing for owner (who doesn't want to say "designed by the newest associate of famous architect X", for getting into ASGCA, for getting started on your own, etc. I doubt those sometimes conflicting needs will ever be fully rectified with some all encompassing system, although many will propose such systems, because maybe they don't have enough to do?