What is an ending?

Stories have a beginning, middle, and end.  US comics seem to lack the end part, until they get cancelled.  Why do most manga and anime, even if they go on forever, eventually end? 

Heaps of US comics end these days. The ones that don't end go on forever because they've spiralled out into something beyond just one story. They're all sorts of movies, videogames, tv shows, spinoff comics. There'd be no point to ending Batman to the point where no Batman product is ever made again because there is so little continuity between them that you wouldn't be able to find an ending that fit. You could end individual Batman stories though, and that happens all the time.

The sad thing is that most of the money in "comics" comes from other media. Marvel comics is now Marvel Entertainment Group and their mission statement doesn't even mention comics, just the merchandising and licensing of "character based entertainment". And they probably made more money from the unsuccessful Daredevil, Punisher and Elektra movies than alot of successful comics put together.