Is it... just me that spends hours on the story, then?
As a distinct measurement of time, no, it's not just you. But it depends on how much you're getting accomplished in those hours.
With the example I used above, those sentences are pretty much all of the "story" that needed to be written for the Garamsythe Waterway. I would daresay that if it took you hours to create that plot synopsis for one of your dungeons, that would be unusual.
Within the sewers themselves, there are 5 scenes with any dialogue, which I estimate don't total 5 minutes to be read out loud. That's not a lot of writing, but if your creative process led you to take hours to craft those minutes with just the right wording, and having each character keep their unique voice...I wouldn't look down on you for it.
But I also wouldn't see why it required a team of writers. Say all of that was true, and it took you several hours to write the sewers. That's a portion of one day's work to write one of the game's dungeons. That means you should be able to knock out several in one day, and it takes a lot of days for professional games to get made.