I pretty much only watch Mortismal Gaming at the moment, who 100%'s every game before reviewing it, but I think with a strategy like that you're highly biased into playing games you're already going to enjoy, so it's not great at finding which games are bad. I believe he has also quit games in the past if he was not enjoying them, and then didn't review them as a result.

I think there are also other reasons that reviews trend positive - it's probably better for ad revenue/engagement to prey on people's hopes that a game is good, and people like for their purchases/prepurchases to be validated. Not as many people will stick around through a 30 minute review of nonstop negativity.