Because you won't

The storyline page sounds like it would be perfect for "plot holes" or story inconsistencies. Why wouldn't it be a good place for it?

Just because a page is incomplete doesn't mean you can't add minor details to it. The policy says:

“	However, you are and cannot be held responsible for completeness; it is the job of the community at large to strive for completeness.	„

and also

“	You may not completely remove content from a page that is in accordance with our policy, however this does not prevent you from presenting the data in a different or more summarized way so long as all the content is still represented in some way.	„

It seems like it'd be better to have the content somewhere, even in a page that's already messy, and personally improve on it, or improve on it as a community.

The page is named Mabinogi Storyline Recap.

The point of a recap is to super-simplify/summarize the story from the content given. This does not include minor details, and as such, wouldn't really fit in. If you're going to add minor details to something that should be straight forward and simple, then it's not really much of a recap.

That is a good point. What if I began working on more detailed pages for individual storylines? I am willing to take responsibility, if you think in such a scenario these plot inconsistencies would have a place.