That's not exactly true. There are no categories for fetishes in the same vein that there are no categories for phobias. It's just that people have gotten so use to generalizing EVERYTHING that they have made a fetish mean roughly the same as a genre. But every single fetish is a singular thing. There are no groups.
Of course fetishes are categories. Nouns exist in order to qualify, to categorize. Fetishes (as in the tags, I suppose that's what we're discussing) are a categorized phenomenom that has many specifics.
I like incest games for instance. Incest is a huge category and comprehends a lot of cases and interactions. You can have incest with mothers, sisters, cousins. Even within a specific character type there are lot of different iterations like elder bitchy sisters (Man of the House, Summertime Saga), elder responsible sisters (Big Brother, Long Live the Princess), etc.
Obviously I don't like them all equally, for instance I wouldn't want anything to do with grandma grandson incest. The point being that, there are fetishes -- categories -- I wouldn't want anything to do with at all, no matter the specifics, the same way there are literary genres I won't touch no matter how good the specific book is.