And They Lived Happily Ever After

Most cinema-goers don't like depressing endings. That's something that Hollywood has discovered over time. The cinema going public would like to leave feeling happy and good about themselves; rather than wanting to kill themselves after the film has finished (i.e. Never Let Me Go syndrome).

Thusly, most movies nowadays must conclude with everybody completing their adventure both better off than they were before they started and with all possible threats completely defeated so that there is no chance that anything bad can come back to bite them in the ass.

Disney in particular have perfected this technique with their animated movies. And they're not even above changing the ending that the source material had in order to make this cliche stick.