Because a very vocal number of gamers who spend their time online are very immature, sadly.
A movie can get very different scores from different critics because it is accepted that a review is necessarily subjective to the reviewers taste. However, internet gaming trolls can't see that with game reviews, and they always demand a game to be reviewed by some super-objective standards (which are none other than their own subjective standards).
That's why I hate when someone says something like "it's unfair to review Uncharted for what it's not. It's not meant to be an open-world game you $%&%$". So what? It's the reviewer's taste, live with it. A movie reviewer that loves drama and high-quality acting is gonna give Avatar a shit score, but you don't see him getting bashed for 'not reviewing the movie for what it is'.
No troll is too much for me to handle. I rehabilitate trolls, I train people. I am the Troll Whisperer.
