Because the user reviews too often don't reflect the reviewers actual opinion. Users have the luxury of anonymity and don't have a reputation to protect, so they often try to 'meta' the score by giving preposterously inappropriate scores to change the average. Anyone with a shred of intellectual honestly will concede that Fallout 4 is an impressive achievement with massive amounts of content. Even it's harshest critics, disappointed that it wasn't what they wanted it to be, will still concede it wasn't a bad game, just not the game they thought it should have been.
"Fallout 4 is a good game, it's just not a good Fallout game." is a common observation among the disappointed. An honest critic who dislikes the game is still going to give it at least a six out of ten for the sheer scope of the thing. But disappointed users and haters are prone try to hurt it's overall score by gaming the system and rating it with dishonestly low scores like 0, 1 or 2 in an attempt to bring down the average. Those aren't reviews, they are attacks and should be dismissed as such.