That is a really good explanation. Still don't excuse the poor directing and poor acting of most of game adaptations tho.
If we look at the vice versa situation - movies adapted to video games - it has been very common that many corners were cut during development because the movie license itself costed a lot of money. That's why most video games based on movies have sucked. The quality of the game was of little concern, because fans of the movie would buy the game anyway.
If a movie studio pays for a video game license, it's probably not too different. They settle for mediocre authors, directors and actors because fans of the video games will pay to watch the movie anyway.