BC hit the nail on the head here. And no, I don't think 90% is overstated, in fact it might even be worse than that.
Yes, because it doesn't matter what the game is like or what its reviews are like, fans are still going to buy it. They might complain about it, they might play it for an hour then never again, but the money will be spent and that's all that matters. It's the same way in reverse (Game to Movie). The game could be a smash hit, but the movie will most likely suck, but the fans will still go see it. On top of that, there can be really great games and really great movies, but the two just don't translate into the other very well.
Here are some explanations why Games after movies "Suck":
1. If a game is made after a movie, the game has too little detail, making it "suck". the only exception I can make is for a series of movies, like LOTR and Star Wars, they have many movies, so game designers can add more details to make the game better.
2. Most games after movies come out around the time of the movie's release date, thus Game designers have only a little time to make the game, another reason for its "Suckyness"
3. Most games after movies have smaller budgets than compared to other games
4. Most games after movies have restraints, they have to follow the movie's plot, which can make the game "Suck"