Because when I watch a film I want to be told a story that comes from the maker's own vision.
If somebody chops, changes and re-edits it then it's pointless. You may as well play an interactive computer game or something.
Stories aren't supposed to be convenient. If a story has a particular ending then it's usually there for a reason. Change it and you change the context of the entire story. That's not what I think cinema is about.
You're watching somebody else's vision and imagination, not playing a computer game for easy and convenient gratification.
Sometimes bad things have to happen for the story to work, you may not like them but often they are correct and necessary. It works the other way too, a film is designed to have a more upbeat ending, because that's what story the maker wants to tell, it's their story.
If you don't like endings to films then you should write your own stories not try to change other people's work in the idea that you think that you can do better.
Try and change it and it's not the same film anymore, it's a different film. Change the ending of Gladiator or something and you can't say that you enjoyed watching Gladiator anymore, you watched some hacked up bastardised version of it and changed what it originally served to do and what effect on the viewer it was designed to elicit.
Of course there's studio interference which can be involved, but I suppose that's a whole new discussion in its own right.