If every Batman game sold around 6.5 million copies, why would the next one sell 22 million copies?
Good question.
For reference, most Harry Potter and Spider-Man console games didn't sell in the 15-20+ million range until they did. And there's a number of reasons for this.
1. There was more competition for that specific kind of game back then.
2. Hogwarts Legacy and Spider-Man 2018 are the closest we've gotten to fully well-rounded games of the IP. They're sufficiently budgeted and creatively apt enough in comparison to literally everything else before them.
Most of the half decent Harry Potter games were either movie tie-ins or Lego games that still resembled and retread the movies, for example.
The gap is smaller for Batman, I would argue that Rocksteady's version, while groundbreaking at the time, is not actually anywhere close to the maximum potential of Batman as a game.
The lack of characterisation beyond the stereotype for Batman himself, no actual Bruce Wayne presence, each story taking place over one night, Knight's story just being plain bad, and poorly implemented supporting cast members throughout and rote gameplay systems actually put a hard cap on sales potential. Some people here will not like me saying heresy at the alter of Arkham, but there has to he some explanation to the fact that you've pointed out. Even "Arkham", being used as the series moniker might be limiting.
You want a Batman game that will sell 20 million, you need to elevate beyond what Arkham and Insomniac's Spider-Man have done. Make it Reverse GTA. The ultimate Crime-Fighting sim, with a high quality cinematic story that rounds out a high-effort interpretation of Batman, Bruce Wayne, his supporting cast and their place in the wider DC universe.
Systemic Gotham that uses an artstyle that doesn't just make it look like Chicago-York or a barely coherent amalgam of city centers like Knigh had it, where civillains, police, criminals and villains interact freely. If it hit all the right markers, forget 20 million, you're looking at 25+ at minimum.
Of course, this requires a lot more time and effort than what executives
think live services do, and the revenue potential is not the same as that of Fortnite or Minecraft or Apex Legends or whatever.