Movie adaptations of games have to work with weak source material as far as stories go. What makes games interesting is the interactivity, but with that being removed for a movie, there's only a shell left.
Movies about the topic of games aren't bound to any source material, so they are properly written like actual movies, hence the notably better results.
That is a really good explanation. Still don't excuse the poor directing and poor acting of most of game adaptations tho.
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