What is a Screenwriter's Job? - May not be what you want.

A screenwriter's job - Someone might say it is: "to build a world and fill it with characters."

I would say it is "to create a character and unleash them into a world."

To build a story world is to be a production designer.  It is to be an art director.  It is closer to pornography that screenwriting.

However, many writers may be drawn to Building their own Universe.  With lots of cool shit.  Or lots of nice set pieces.  Or lots of beautiful costumes.

But a SCREENWRITER.  Takes a character.  Applies an incident.  And develops that characters reaction to that incident into a goal and Story.  What the story looks like, the framing of the scene, falls in line like the last pieces of a puzzle.  Child's play.