The question is manyfold. Why are there so many. Why are they shown, when you open browse. And are they really popular.
The second one is easy to answer. The initial sorting is by popularity, whatever that exactly means. Horror is popular by the standards of measuring such a thing. If you sort by ratings, it looks less horrifying, but still 9/36 have the horror tag on page 1.
Why there are so many horror games, is probably a combination of many things. Like, the self fulfilling prophecies. New devs see horror games are popular, so they try to make one, adding to the pile. And since they are shown on page one so much, people click them and they stay popular. Also, "horror" is dirt cheap to create. Put in a jump scare, and a creepy image and you can claim it is a horror game.
So, are they really popular. Yes and no. No in general gaming, and among itch users there are many that deem them annoying, precisly because they are in abundance.
But among indie games as a whole, oh yes. They are popular. Big time. Think Slender Man games and of course Five Nights at Freddy or Doki Doki Literature Club. Not to diminish successful games, but I think that of all the genres, it is the easiest to make a good horror game with shoestring budget and limited skills and game mechanics, so it is just a good fit for indie development. It is easier to shock, disturb, creep out people, than to write good comedy, engaging story, eye candy, game play, level design, puzzle desgin, etc. And with the good games there come along all the other ones, that try, but do not quite get as successful as others, but still get enough clicks to be "popular" according to the algorithm.
(Oh, and if you want to filter them out, that is possible. Add
?exclude=tg.horror