Well, they did that
because they have plans...
You know, the first Spider-Man
film tied Spidey's origin to Oscorp, and the first Batman
movie had the Joker as the guy who killed Bruce's parents. There's a dramatically sensible tendency to more tightly integrate important aspects of the continuity when adapting a serial medium that grows by improvisation and accretion into a narrative that will run, at most, about two and a half hours - hence making the Guardians responsible for so much in the movie. Even in the comics Johns and others before him had already retconned Sinestro and other more recently created GL Corps members into Hal's origin story.