It's entertainment. People watch it to enjoy themselves. It's a different kind of entertainment, but in both cases you're not participating, but passively looking at what's playing out in front of you. Clearly these people are as invested in these films as you may be in United's fortunes. Obviously, they know it's going to end well (they have that advantage over you), so the interesting part is how that will come to pass rather than whether it will, and what happens along the way. But the underlying point remains that they watch this for their fun.
The problem is just that people look down on that because it's not what they want films to be. It's like me looking down on pop music drivel. (Which I do and shouldn't. I've gotten better over the years though.)