Horror movies as a genre includes a large portion of direct to video/direct to streaming lower budget films. If we look at action movies and include all the Stephen Segal movies, or romance movies and include all the shlocky Hallmark stuff, or sci fi movies and include all the low budget crap like the Asylum puts out the ratios for those genres are pretty awful too.

Horror just happens to be one of the few that include non-theater releases in what people tend to think of when discussung the genre because of being excluded from theaters for so long. Comic and superhero movies had a similar begatuve popular opinion because of how bad most of them were before the 2000s.

90% of anything tends to be crap and always has been.