That would be my "Break canon for the sake of gameplay" suggestion, which I still think is the best option to make a good game out of a property.

Maybe I should say that my point against this course of action isn't so much that it's bad, but more that I don't believe devs have the guts to go through with it.

Just look at Shadow of Mordor. It stretched LotR canon pretty goddamn far, but they never had the guts to just have you straight up fight Sauron. It would be a better final boss than what we got in the game, but they just don't have the guts to so blatantly go against the canon. In that sense your idea is good, but answering the original question of "Why don't we have a AAA game based on GoT?", the devs don't have the balls to break canon, or the rights holder (I assume HBO, I don't think Martin would care) is too in love with their own canon to let someone else break it.

I think it was the 80s and 90s when they'd make games out of movies or TV series. Quite often these were the worst kind of games, poor playability, riding only on its name.

Nowadays it mostly goes the other way, they make movies out of games.

I have to admit that I have no interest in buying a game just because it is based on some movie or TV series I like. So let's say they make a game called The Big Lebowski or Seinfeld... nah, not interested, unless it they started getting glowing reviews.

What about Mortal Kombat film, its cracking and that was 90's. TRON as well, can't remember if film or game came first there.
Sure Street Fighter was dreadful.

I think it depends on both the chosen movie / TV series that they're working from, and also whether they're using the original characters or are using the same universe and society as a backdrop for a new story and characters. And other details, of course. To me, a Firefly-based game could be great, using that background but with characters other than the show's cast. Maybe some cameos, but I don't think I'd try to recreate the experience with those characters.

Fortunately, Whedon was creating a world with a lot of character of its own so the game would not have to rely on the original cast.

There is a Game of Thrones mod for Crusader Kings II that is acting as the "unnoficial official" game. It looks really good and it was actually how I learned about CK in the first place, so I guess it brings visiblity to both franchises.

A Firefly game would be awesome if the original cast was involved.