I can see why a critic wouldn't want to finish in certain situations. If you've played for lots of hours and are hating it, what good would spending more time with it do? It would probably make you hate the game even more, and would be a waste of your time. Plus reviewers are probably spread thin now, with so many good games coming out in such a short time frame. I don't know, if you have a requirement that you ALWAYS need to finish each game you review, you would sort of get an unrepresentative slate of reviewers who are all at least somewhat competent with the game, and a self-selection of reviewers you would expect to enjoy the game. This is not reflective of the entire game-buying public, and the role of a site like Metacritic is to collate all of these different points of view on a game.
That said, he specifically called out the story for feeling incomplete, and that is fucking stupid if you know you have barely scratched the surface. A smarter approach would have been to focus in more detail on what, specifically, pushed you away from the game