I believe the root of the problem is how you browse for games on each platform. Steam does more promotion of games. Or do people really go to Search games and leave all the parameters empty? Because that is "popular" section on Steam and there you can see the current most popular titles of their ~ 160k games. I did not even know that section existed for years! Let alone that there is a not very visible button to minus tags, instead of checking them to filter with.
The browse section on itch is much more volatile and fickle. Smallest trends among youtubers will be visible there. And there is a certain promotion of new games in the ranking. Both those factors increase the visibilty of "horror" titles.
Whereas on Steam you have a huge landing page and see promotion and tailored selections to your previous tastes. There is a recommended section on itch and in theory the landing page. But after making an account I basically never visited the landing page on itch. Like, never.
Regarding horror specifically, Steam implemented 3 or 4 matury settings and those settings are curated. You should be able to block them based on maturity. If you did not do so and still do not see horror on Steam, that is because it is not promoted to you.
In other words, are you flooded by ugly horror titles in your recommended section on Itch?