Well, I personally was pleasantly surprised by the Assassin's Creed movie that they made, and also by the Warcraft movie. They weren't great pieces of art, but they were fun in their own way.

As to your questions:

1): Partially it's marketing. As of right now, if someone made a movie based on Metal Gear, Halo, etc (just examples, really any game)., who's gonna watch it? You could cater to the fans, but that's not gonna generate that much money probably. Movies want to appeal to the largest demographic possible. Making a movie about Halo (for example) that's solely dedicated to the Halo universe faithfully isn't going to capture a very large demographic. Some people are gonna see the Halo title and immediately see something not worth seeing. For others, it has to be appealing in order to be worth watching. So people try to change the whole film to create mass appeal. Many times the result is a dumbed down movie no one wants to watch.

The other reason that many people cite is just how different video games are from movies. Movies are a lot more like books, which have a linear progression to a story line, 3 acts, etc. Games are meant to be immersive and interactive. Movies aren't meant to be interactive. There's also the argument that games are not really like other mediums of artistic expression, which I'm not sure I really agree with, but there are clear differences.

Comics have been and are being adapted, and for a while, only had poor adaptations. Certainly comics are also different from movies, but they are still not that different. Maybe games could get something similar soon once people figure out how to do better adaptations. Leading me to your next question.

2): I can see it happening eventually for certain games. Mass Effect and Metal Gear, for example, could really work, since they rely heavily on story telling to keep the player interested. Skyrim, for example, would be a bad idea, however, given how the whole point of the game is to chart your own story. League of Legends would equally be a bad idea, as would Call of Duty. I can see a good Starcraft movie or a Halo one too.

 In the next 10 years? Maybe. There is a lot of interest around video game films recently. There probably is a good way to translate these stories to movies, people just haven't done it right yet.

I feel like the earlier Tomb Raider films (with Angelina Jolie) were at least somewhat successful? Rotten tomatoes has them at 20% and 24% respectively, but I don't remember them being quite that bad, and both appear to have made a decent return on their investment.

Resident Evil continue to be good mindless fun in film form (though I can't say I've ever spent much time with them in video game form).

I think at least part of the problem is that AAA titles are almost universally the ones adapted to film, and those don't tend to be particularly good at storytelling. While there's a ton of really interesting storytelling going on in smaller and indie games these days, AAA tends to run the gamut from overblown-Tolkien to schlock-space-opera, by way of cheap-walking-dead-knockoff. And that's when they aren't just trying to remake famous war movies in first person...