Because some people just can't do it.
On the other hand, some writers/mangaka have sharpened making shit up as they go along to an art.
Take Haruhi, for example. Tanigawa states that he never really had a plan at all for the first several books, and just wrote stuff that seemed fun. And people ate it up.
Then, there's stuff like Dragonball Z that was forced to change, against the mangaka's wishes, so he had to royally ditch his plans.
Sometimes shit happens, I suppose.>>13613133
Oh god, best answer ever.
I love Oda's reader responses.