Affleck probably wants to hand over the stress and responsibility of handling the overall movie to someone else.
Kevin Smith’s comments on an episode of Fatman on Batman arguably seem like the most logical and reasonable reason for why Ben Affleck decided to take a step back from the director’s chair on the upcoming Batman solo movie.
That way he doesn’t have to shoulder the responsibility if the film isn’t good.
It’s a pretty blunt rationale, but it also makes plenty of sense.
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AFFLECK WILL NOT DIRECT THE BATMAN - KEVIN SMITH'S THOUGHTS