To be fair, these movies rely heavily on fan support and thus it ends up driving certain behaviors. One, you get a movie with minimal plot or at least a shallow one since it's either largely based on the game, which was shallow to begin with, or you get one that was created just for film, which features only limited elements from the original game. Second, you tend to get a movie with very limited budget since the movie studios have seen that such movies don't perform too well at the box office and so they'll want to minimize their exposure while hopefully maximizing potential revenue. Ironically enough, these behaviors only continue to drive the poor performance of these films at the box office and one can only hope that sometime take that gamble and go all out in support of a video game movie.
Yeah I know, keep dreaming, right?