Stargirl did a better job of establishing its universe in 13 episodes than the DCEU did across three films. It accomplished this through a greater focus on characters over plot and taking care not to overwhelm the viewers with cumbersome continuity.
While contrasting television series and films is generally an apples-to-oranges comparison, it is apt in the case of Stargirl and the films of Zack Snyder. Both took up roughly the same amount of time, if one factors in the extended director's cut of Batman V. Superman and the projected length of the upcoming Snyder Cut of Justice League. However, Stargirl did a better job of establishing its reality in the early stages than Man of Steel did and grew stronger with each successive episode building on what came before. The films also tried to build upon the foundation of Man of Steel, but ultimately seemed more concerned with the details of building to the next story rather than developing the characters.
Stargirl, as overseen by Executive Producer Geoff Johns and a team of writers including comic book legend James Robinson, remained firmly focused upon its characters throughout its first season. The two-part pilot episode established the sort of people Courtney Whitmore and Pat Dugan were long before cosmic rods were ignited and robotic armor was donned. The rest of the cast was slowly rolled out in the same way, with the future JSA being introduced long before they donned costumes or got into a fight. This resulted in a much more satisfying finale, as all the separate threads came together in a gloriously woven finale that offered a satisfying conclusion but also hinted at what was to come.
By contrast, Snyder's films are more focused on plot and building the world over the characters. Many of the cut scenes from the cinematic release of Batman V. Superman involved explaining the mythology of the New Gods and setting up the appearance of Steppenwolf in the next film. While this sort of minutiae may appeal to comic readers, most of it sailed over the heads of the rest of the audience and only served to slow down the film's pacing.
The focus in Snyder's movies also tends to be on the action of the moment rather than the motivations of the characters. For instance, the appearance of Wonder Woman during the battle with Doomsday in Batman V. Superman was a brilliant piece of cinema and many singled out Gal Gadot's performance as one of the best parts of the movie. Yet, the greatest action scene of the Wonder Woman film, in which Diana mounted a ladder and ventured into No Man's Land alone, proved to be a stronger character moment in addition to a better action sequence precisely because of how the character was built-up before that point. Batman V. Superman may have told viewers who Wonder Woman was but Wonder Woman gave Diana the chance to show us who she was.
It's possible the Snyder Cut of Justice League may redeem the general public's antipathy for the DCEU. Yet one can't help but wonder, based on the success of Stargirl with fans and critics, how things might have been if Geoff Johns had focused on films and developing the DCEU instead of turning to television with his production company Mad Ghost. Ultimately, it is enough that such talented creators are all getting their chance to play in the sandbox of one of the greatest shared universes in all of fiction.