Because to rate a game, you need to write a review. And the reviews are moderated.
Example if a game is advertised as an incest game, but 95% of the content is some futa NTR gangbang with unrelated people, you can't just write "this game is a fucking fraud, 0/5 rating" but you have to rate the game as it is.
Or if the dev is clearly a patreon milker and a jerk, you also cannot write that.
So basically the rating scores on this site are useless, since they don't represent the consensus in the community.
If it wasn't moderated, all games with NTR would be reviewbombed by harem-only supremacists. Now, when I even see NTR mentioned in a bad review, I automatically assume 99% of the reason you gave it 1 star is because you're just an NTR hater and I'm not even gonna take you seriously.
Having it unmoderated would tell us absolutely nothing about "consensus in the community", because people don't hate on vanilla/harem games but their fans often like to hate and shame all other fetishes. That would make it seem like most of the site only like vanilla/harem games but that is not the case, they are just more vocal in their hate for other tags. In reality, most people here simply don't care about fetishes they are not into and lots of people like things that are not vanilla.
I am not saying this moderation is perfect but you should realize that rating score would be be even MORE useless if it was unmoderated.
Maybe if we didn't have a bunch of babies on this site who can't cope with the fact that people enjoy stuff they don't, we could have a looser review policy.