Because if you're neutral, you're not recommending the game.
There is kind of a disconnect, right?
Steam asks whether or not you’d recommend the game, but then they summarize the review score as “%positive reviews!”
What I sense is that very often, people just want to voice their criticisms without necessarily affecting the game’s score, usually because they appreciate the craftsmanship or genuinely enjoyed most of the experience. If a game left you “on the fence” but still inspired you to write about it, it’s probably because there was a lot about it that was good, except for a few hangups you experienced. That type of review can often be the most insightful
"This game isn't for me, but I can see why others would like it" seems like a perfectly reasonable neutral review. It would also be helpful to someone considering a game outside their usual genres. For example, palworld was very much not for me, but there were aspects of it I enjoyed and I don't regret buying it.
I wish i could leave a rating without the need to write something
If people could post neutral reviews, Steam would still display "X% positive". Nothing would change. Hell, you could let people post neutral, negative, or double plus negative reviews, and at the end of the day, the result is still "X% positive".
If you want any value from the negative reviews, you have to read them. Adding other options doesn't really help. I'm glad people post negative reviews to mention how "blah" a game is. It highlights the review so I can read to see if their problems with the game resonate with me.
In most review sites that have stars, everyone just posts a 1 or a 5, anyway. Or you have those idiots that say "it's perfect, I love it! 4 stars". Then you have those unicorns who post a 3 and really defend it, but it's so rare.
Board Game Geek has an interesting review system where it's 1 to 10, but the numbers are explained with words. This results in a 7 being a really good board game. All review systems need something like that. Words to articulate what the numbers really mean to calibrate people's use of them. Either that or keep it simple like Steam does.
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