Similarly, saying Burton "only" produced Nightmare Before Christmas severely underestimates his role. Only a peculiar notion the director is the main creator could leave anyone in any doubt as to his primary responsibility for Nightmare.
I don't think anyone here has said that the director is always the main creator. It depends on who's involved in each film and how much power they wield over the creative process. As I said before, some films are producer-driven, but films involving major directors are usually director-driven (even on films where they aren't producers or writers - Tim Burton, for example, neither produced nor wrote
Alice in Wonderland).
I suggest it's because Burton's directorial contribution is still secondary to the script.
A director at Burton's level is going to have a significant impact on the script, though. He's going to work with the writer until he gets the script he wants. The writer's skill and creativity are of course crucial elements, but the writer doesn't have the creative freedom to write whatever he or she wishes.
As for consistency in theme and style, some artists - directors, screenwriters, novelists, etc - have a unified vision that carries through all their work, while others explore different areas. The latter approach doesn't negate their artistry and creativity, nor rob them of authorial credit. For example, think of how diverse William Goldman's work has been.
Stepping back from the discussion of major directors whose stature gives them creative control and looking at more run of the mill directors, they're a key element in a film's quality, or lack thereof, in that how well a director works with his cast and crew has a huge impact on how well a script is brought to life. And, indeed, a director's skill in working within the financial limits set by producers and studio execs is an important ingredient in filmmaking, one that can affect the quality of the film. In this scenario the journeyman or neophyte director in a work for hire situation isn't the main creator, but more one of the leading co-creators. A B-level director will generally be left to perform the function of overseeing every production element in trying to achieve a unified vision, but with the important caveat that he can be overruled by the producer in a way that an A-level director usually can't be. The extent to which a B-level director has creative freedom will depend on the producer he's working for.
A point of clarification: I've been addressing the state of the film industry as it is now and as it's been since the breakdown of the studio system. The studio system was much more producer-driven. The director's role was really to, hopefully, provide skillfully staged shots and to draw good performances from his cast. The various elements of filmmaking were more compartmentalized as functions of the film factory. After the studio system broke down directors steadily gained in creative control and took on the kind of "thousands of decisions per day" role they usually perform today in seeing that the various elements of production come together with stylistic and tonal unity.
As for Clint Eastwood, the themes he's most often drawn to are those of a lonely man trying to make connections and give his life meaning through the love of a woman and/or a surrogate family and, later in his career, the enduring effects of violence.