Most people just enjoy good characters. Anybody with any sort of understanding of how storytelling works would know how the end of Jesse and Walt end up in Breaking Bad. They would know how things will shape up in The Wire. Same with Captain America: Civil War with Team Tony vs. Team Steve. And people who read the book could be big Robb Stark fans, know how he ended up, and still be rooting for him. People would also come to conclusion that the Starks will come out victorious and people can still root for someone like Cersie. It's not all that serious.

People just like characters and it is somewhat fun to root for characters despite the outcome.