First of the reasons may very well be the simplest of all: money. Per The Hollywood Reporter, “The Flash” is a $200 million movie, and therefore significantly more difficult to flush away than something like “Batgirl,” which had a budget of around $90 million. As DC Studios executives, Gunn and Safran simply can’t badmouth the movie like that, regardless of their personal feelings … though that’s not to say they’re not telling the truth when they say it’s great.
People who’ve actually seen “The Flash” seem to love it and praise it to high heavens — including Warner Bros. Discovery CEO David Zaslav, who had great things to say about “The Flash” in a 2022 earnings call (via Variety). “We’ve seen them, we think they’re terrific, and we think we can make them even better,” he spoke of three upcoming DC movies. Of course, it’s worth noting that one of them was “Black Adam,” so your mileage may vary. Still, while Miller’s past behavior may very well warrant a whole philosophical discussion about the merits of separating art from the artist, there’s a very decent chance that “The Flash” will turn out to be a worthy swan song for the old DCEU.
As for Miller, well — even if you were willing to disregard their various issues, the fact remains that they’re one of the last Justice League guards still standing. Even if “The Flash” makes “Avatar: The Way of Water” money at the box office, there’s a chance that Gunn and Safran are simply waiting for Miller to recover and recuperate in peace before sitting down to have a conversation that ends the same way it did for Henry Cavill and Gal Gadot.