Because it's still worth experiencing.
For me it's not the events in a book it's the set up, the suspense, etc.
I mean I know the good guys gonna win in most any book I read. No surprise there. I read Sword of Shanarra and then read the prequil, I knew what was going to happen. Didn't bother me none. I knew so and so would die in this book, or the ending of that book, kind of can dampen things but I don't see how it ruins it.
I'm still reading for experience and to get lost in words, not to get to the ending so I can see what happened condensed in a short "so and so died" set up or a "they did this and then that" line.