There are lots of reasons, really. However, the biggest reason is quite simply put, time invested in making the game. Once a production takes off in Hollywood, it takes off very quickly. Let's take a look at something that we know is coming out in the next few years, Marvel's Thor 2 (and no, I won't reference the "game" that came out with the first movie). Part of the production of a movie is merchandising, thanks in part to a gentleman by the name of George Lucas. Merchandising will also include choosing a publisher for the upcoming video game. Video games can work a little differently, though. For instance, Marvel signed over Spider-Man to Activision years ago, and that is why all Spider-Man titles (movie based or not) have all been put out by Activision. In the case of Thor, Sega did have the rights to all the Avenger characters for their games, but we all know how that turned out, don't we? As of right now, Thor could basically go anywhere (I am aware of Ubisoft's upcoming Avengers title, but we don't know if that includes individual hero games as of yet). So shopping the big guy around to publishers will start the ball rolling here. Once a publisher is chosen, then the publisher has to choose a house to develop the game. A larger publisher, like Activision, is more likely to keep development in a house that they own. A smaller publisher, like Sega, is more likely to shop the game around to independent developers. Case in point here, Sega gained the rights to Aliens a while back, and they ended up going with GearBox to develop it. GearBox is independent, and has done games with UbiSoft (Brothers in Arms) as well as 2k Games (Borderlands). While those two steps sound like they should be relatively easy, they are actually the longest part of the process. Getting back to Thor here, after the success of the Avengers movie, I would guess that blondie would be worth quite a bit to the right publisher, so choosing on one can take some time. From there, getting the right house to do it will also take time to settle on.
From there we get into the actual making of the game. This means the developer has to be made aware of the script of the movie. In the case of Thor, they would have to know locations to make, characters to render, and other various parts of gaming development (the score for the game, the voice acting, etc.). The studio needs to know just about everything in order to make the game an official tie in. Sure, there are some games that take place "in the universe of" but not taking place during the movie (such as Green Lantern), most movie studios want the game to be an official tie in, to make the audience play the movie once they've seen it. Then there comes other cosmetic things, such as voice acting in the game. Some games require the main stars to provide their own voices for the video game. In the case of Thor, they would have to get Chris Hemsworth and maybe a few other actors/actresses as well. Hemsworth is a pretty busy guy these days, and getting him to come in to provide voice over work has to be done pretty quickly.
Now, I hear what you're about to ask, "Not *all* movie games suck!" That is correct. There are good movie games. While I do like to use the phrase "every pig finds an acorn every once in a while." There are reasons behind this phenomenon (that's sarcasm). I will use the game Kung Fu Panda here, because it really is a good game, and based off of a movie. Activision did something here that more companies need to do. They didn't do anything fancy. It was a panda running around beating up other animals. Nothing more, nothing less. It didn't need high quality graphics or any other bells and whistles that a Transformers game would need. It also did not use any of the voice talent from the movie what so ever. It was just a well made beat 'em up that was fun to play.
Other "good" movie games are often based off of movies that have been out for a long time, such as Ghostbusters. The Ghostbusters game from a few years ago was a title put out by a company who really loved the Ghostbusters, and put their heart and soul into making a good game. It didn't hurt, either, that Dan Aykroyd and Harold Ramis took an interest in the project and personally brought in all the talent needed to voice their original characters (which is saying something, considering how Bill Murray can be). This game had no time table that it had to be out by because there wasn't a new movie coming out. It was merely a game being put out by a bunch of people who loved the Ghostbusters. The final result? And excellent title, top to bottom. Most of your top notch movie titles are examples of this. Goldeneye 007 on the Nintendo 64 was actually brought out the summer before the following Bond movie, Tomorrow Never Dies. The result? Well, it's a legendary title that shaped the console first person shooter into what we get on our consoles these days.
The only real anomaly here is Star Wars. Yes, I know not all Star Wars games are good. In fact, Star Wars: The Force Unleashed II was an embarrassment on levels that I will have to get into one of these days. However, most Star Wars games are actually, well, games. They are playable, and you do have the ability to have fun with them. While they do fall into the Ghostbusters camp more than the Thor camp these days, I do believe it is important to point out that George Lucas owns his own video game development studio and publisher. Pretty sure that helps produce a decent product if you're trying to get a movie out. No one controls his intellectual properties better than George Lucas, and you can quote me on that one guys.
Overall, the truth is that movie games tend to suck because they don't get the same love and attention that other games do. Sure, someone might love the idea of making a game to go along with Thor, but unless the correct amount of attention is given to the game, like most games get, then it's going to be fighting an uphill battle from the second that it hits the shelf. Reviews will be mediocre at best, and there will always be some other game out there that gets higher scores around the same release date. Sure, you'll still have the kids coming in to pick up the title after seeing the movie. Wouldn't it be nice though to have a movie game that makes you want to see the movie again though? Unfortunately, that tends to not be the norm when it comes to video games based off of movies. I would say, "let's hope that changes," but as long as the Hollywood way goes, it's never going to get better.