In order to review a game the reviewer needs to experience all the aspects of gameplay. In some games, you need to finish the whole game, in others, not so much.
With Bioshock for example, you could get all aspects of the actual gameplay fairly early on and review it based on that fairly well, but you would be missing out on the story which is important in that game. So for a game like Bioshock, you need to finish in order to properly review it.
With a game like Dewey's Adventure you can play it for 2 hours and get all the gameplay. You don't need to play more to review it because there's nothing new introduced and there is no story element that really affects how you play. Even with the different boss and level puzzles, it's still just the same basic gameplay. I'm not saying that's a bad thing, in fact it's a great thing to have simple systemic gameplay that works well throughout the entire game, I'm just saying you can play a few hours of some games and get everything out of it.
