Because investors don't care about the quality of a product as long as it returns a profit. As shitty as the film was, The Happening actually returned a sizable profit.
Also producers and investors don't really care so much about the quality of source material as much as they care about its target demographics and its spending power. Avatar: The Last Airbender, regardless of how well written the animated series was, is considered to be a draw to small children only who drag their parents to see crap all the time.
