Yeah, I still haven't gotten used to America Graphic Novels. The one I'm most familiar with is Sin City by Frank Miller.

It's a time issue. Some Japanese customers may only purchase Jump magazine to read two-four issues of manga they're familiar with, but just like our American comics, their mangaka only manage to release singular short issues with the constant "To Be Continued..." at the back. When I read Shonen Jump magazine, there are at least three issues of any given manga included in the book and when I pick up a manga for, say BLEACH or Naruto, there are like six-eight issues bound together.
Did you know there were 12 issues of Watchmen which make up its series?* And they bound them all together into a sleeve no thicker than perhaps two volumes of manga paperbacks would be combined here in the states. Of course, the size matches the traditional American comics sleeve in length and width.
*The same number makes up the DC universe simplification series "Crisis on Infinite Earths."