You're not really thinking this through enough. The exact same logic you used can be applied to music: once you've listened to the album once, you know how it will go, so what's the point of listening again? Presumably you do get something out of listening to music again (since you said you do listen to it more than once), so whatever that "something" is... you can infer that others get similar value out of rereading books/rewatching movies, even if you personally don't.

For myself, the answer is "because the story is still enjoyable even if I know how it will end". And often enough, on a second reading/viewing I will discover nuances to the work I didn't the first time. Some works are so well made that even having enjoyed it 10+ times, I can discover something new about it! So yes, the pleasure of experiencing the story the first time can only be had once. But that is by no means the only pleasure to be had.