1. The designer couldn't think of a better way to tutorialize the game concepts, so now we have to have a character explain them all to you in exhaustive detail. (See also: the writer; lore.)
2. The creative director was checking emails while playtesting and got confused halfway through the quest. He demanded that the writer write additional dialogue to periodically recap the plot. (See also: the game is designed to distract the player, so that they actually *can't* focus on the story--i.e., most open world games.)
3. Players got lost on their way to the quest destination, so the writer had to repeat the name of the quest destination six times and explain how to get there.
4. The quest destination changed, so the writer had to explain why the NPC couldn't just tell you where his house is.
5. A critical part of the level was cut, along with critical scenes that demonstrated the characters' motives. Players then got confused, so now we're "fixing it" with text by having the characters explain their motives.
6. The level designer was relying on players finding dozens of hidden books and reading all of them to understand what was going on. The creative director found none of them, got confused, and now wants it all explained in dialogue.
7. The writer only had four hours to write this quest.
8. The writer thought they were being clever.
9. The company didn't think hiring an editor was important.
10. It's not too long; the game team is not representative of the target audience.
Look, I'm not saying that writers don't have a tendency to get high on their own supply, but every single item on this list (including the last) has happened to me in my career. Even the shortest dialogue is too long if the content and context are not engaging, and many long, dull dialogues are the result of endless fiddling and kluging.