When the film has two, or even three, big stars, they all may want to do this. Thus the multiplicity of producer credits. 

When it comes to writers, though, the situation is different and more complicated. The Writers @Guild of @America allows only three writing credits on a feature film, although teams of two are credited as one, separated on the credits by an ampersand ("you & I"). However, if each person works independently on the script (the most common system), the two are separated by an "and" and credited as "you and I."  But wait! You wrote the story on which the script is based, so you get "story by" credit, and your credit for the screenplay precedes mine, even if I wrote most of the script, except that if my script made substantial changes to your story, I'll get first "screenplay by" credit. If more than two of us worked on the screenplay, the credits will probably read something like "screenplay by you & I and he and she." You and I worked as a team, but he and she worked separately. It actually does have a certain logic to it, when you think about it. After all, the movie "The Flintstones" had, by various counts, at least 35, and possibly as many as 60, writers who worked on the script. Somehow the system found a way to not list most of them, and for that we can be grateful. 

The @Directors @Guild of America permits a film to list only one director, even when it's generally known that two or more worked on it. Except in very rare cases (a death in mid-production, and it had better be in the very middle of mid-production) there is only one directing credit. This is very good for a director's ego, certainly for the one who gets the credit, but also for the one who doesn't -- particularly if he or she had started production and then been removed by the producer at the insistence of the star or the studio.