So a satisfying green screen appears and the user congratulates herself -- she has finished reading all the tweets, all the news. But that's not possible, is it? Some tasks don't have a satisfying end. So introducing an arbitrarily created one just calls the whole task's meaning into question.
This idea was initially used well in Mailbox as some people do finish triaging their email. But even there completion congratulation had its problems. It was always quicker to leave mail in the inbox if it was needed later. That meant processing it just for the sake of 'completion' wasted time, and ruined the associated sense of satisfaction.
All that's not to say infinite scrolling is always perfect UX. If done without some signposts, or other way for the user to get oriented, infinite scroll can be terrible -- like infinite pagination.