Is it because that whatever movie is delivered it won't be the movie that people had in their head of it should have been?
Resident Evil is the most (in)famous example of this, I think - fans of the game hate it, but it has made so much money on the Alice premise ... and she was never a character in the game. That said, she could be now - I've never played the games but I love the movies.
You're right in that it does seem to be a particular thing with games, though. Book adaptations seem to fare pretty well (by and large) but game adaptations are nearly always lambasted - even though the "book in your head" is always going to be different to whatever they've put on screen. (In some cases the film is only vaguely the book it came from - "The Running Man" springs to mind. I'll confess to never having read "Do Androids Dream of Electric Sheep" - but I think the book is quite different to "Blade Runner").
I don't know about the movie people playing the games, though - apparently Anderson was totally obsessed with Resident Evil and his impassioned pitch to CapCom was what got him the gig - and Cap-Com themselves seemed to be happy with what he's done (given it has five sequels).
On the other hand, I'm sure Uwe never played any of the games that he's adapted.
It's an interesting question, man - I just don't know why they're so received so poorly all the time.