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Do you think these hentais are competing to see who can have the longest name?
Real answer: this is a solution to a problem that started with light novels and manga. They have blurbs in the back describing what's in them, just like Western media does, but nobody was reading them.
The solution was to make the title as obnoxiously descriptive as possible, so people would know what it was about just by reading the title.
This naturally affects animated works based on those book titles, and hentai is no exception.