That is because in general the tie-ins movies/games are pure garbage rushed out. But there have been X-men games at SNES/Genesis and perhaps even before, and well X-Men comes from comics and not movies =p . Also it is Insomniac and they done great with Spider-Man.
And yes most movies adapted from games were kinda trash as well.
What I see is that most of those from both sides only want to use the IP name for a quick buck so they don't really care about the quality, but when they do the result is satisfactory. The original Mortal Kombat was a great movie for example.
Yes, but it's natural to have some bias to the form we're used to. I never read an X-Men comic and so movies defined the series to me. I also think that novels, comics, movies, TV series and animations can translate to each other more faithfully and easily than to video games due to their shared lack of interactivity (comics to animation being the most graceful transition of the bunch); gaming is a very distinct medium. Competently turning an existing novel, comic book, movie etc to a video game or vice versa is a unique challenge to undertake. Then again, Marvel works are pretty flexible, retellable, and transformable, so this doesn't necessarily apply here. I can see why everyone is excited about their favorite superheroes translating to top-tier video games.
In most cases, the original form of a good work of fiction is superior and "more accurate" to its adaptations (regardless of the budget or the group of people involved in said adaptation), but the form we're accustomed to (like me with X-Men movies) is often what defines the series to us. Personally, I like the vast majority of works of fiction best when they're remade, retold, or continued within the medium that introduced them to me. This medium is more often than not the original work (Not so with X-Men :P)