Because then you remove the punch of finding it out over the course of the game/movie/tv show/whatever. The point of plots is to build up to some big reveal, and you just read on the internet "Thanos snaps half the universe at the end of Infinity War", then there's no point to watching the movie, because you know what's going to happen.
It ruins the experience.
I'm gonna sound contrarian, but I'm totally fine with spoilers usually. Leaks in general are practically spoilers when they're right and sometimes I'm playing a game because I know what to expect like Persona 5. I wouldn't have played Celeste, for example, if someone just told me it was just about a girl climbing a mountain. Sometimes, it's more about how the characters reach the twist than anything.
That being said, though, Undertale and Infinity War would be completely different experiences if I knew what to expect from them. The entire appeal to them is that we don't know what to expect going in. I've been waiting over a decade for Thanos, so I'm not spoiling myself on this ****. It's closure time, baby.