Guy writes science fiction screenplay. Takes care to tie up loose ends and create a coherent universe.
Studio brings in script doctor who punches up the dialog and adds explosions. Drops some of the explanations.
Script gets rewrite by big-name writer who adds his signature twists. Drops even more mundane “sciency” stuff.
Director shoots the film, concentrating on the action and letting the second unit film the expository talking.
Editor cuts the film down to 98 minutes. Has to cut out a lot of talking. Keeps all the explosions.
Viewer sees movie. Wonders what the hell just happened.