This is one of those odd regulatory things where it doesn't make sense unless you know some of the history. At some point, there was some dispute about how much contribution entitles one to be in the credits. So to avoid that problem in the future, they do it this way. This also makes a bit of sense when for screen entertainment, the plot, development, and such are done by one team, and the actual writing (dialog, actions, etc) are done by another team. The people who are highly creative aren't always the ones that understand how to make a movie, and vice versa.