A message from from Shelly Goldstein reminds me that Richard Gere won the Golden Globe for Best Actor, not Best Supporting Actor. And Joe Creig writes to ask why the Academy doesn't set up rules as to how much screen time denotes a Lead Performer as opposed to a Supporting Performer. I suppose there are two answers to Joe's question, one being that it's a subjective distinction, and any firm number is going to be arbitrary and arguable.
The other reason is that the Oscars — and this is true of the Emmys and the Tonys and any of these — exists because the Hollywood community wants to give itself awards for reasons of ego and hyping the box office. No one wants rules that might exclude them from winning. In fact, the relevant committees that govern each award are constantly being petitioned to loosen things up and give entrants more latitude — and they usually say no. The administrators are generally afraid to make any decree that might cause some influential industry figure to scream that they lost an award because of a rule change. Unless there's a huge outcry to fix a problem, they'll leave things as loose as possible. That way, when someone doesn't get a trophy, it's because of the voters, not the rules committee.