I think the rise of "spoilers" has something to do with a shift in literature (and film) from "journey" stories to "destination" stories. Everyone knows that Oedipus kills his dad and marries his mom. Everyone knows Romeo and Juliet die in the end. Everyone knows that Frankenstein and his monster don't really get along that well.
And yet we still read (and watch) these stories. In fact, in ancient Greek plays, the chorus would inform the audience beforehand what was going to happen. The audience was far more interested in how the characters end up at the places they know they have to.
But nowadays, it's usually the end of the book/movie that is most interesting. We find the climax and the reveal more interesting than the setup. We like being surprised by where a story leads, and get upset when plot points are either predictable or blurted out to us by those who've read/watched it already. Perhaps we can blame M. Night Shyamalan for this (I'm sorry, but aliens who are allergic to water just so Mel Gibson can be a priest again is the stupidest thing ever), but this is a observable trend in modern media.
However, this can be overturned again in books and movies start with the ending, or at least with characters who reveal their ending, and then we just have to keep reading/watching to see how they get there, or if they're just unreliable narrators. I'd name examples, but hey, spoilers.