Because it's really hard to do right. To find that balance between the enemy that is completely unaware you're killing his buddy 2 feet away and having the enemy share a hivemind so if one of them sees you in a bush the entire map knows your exact location instantly.

When it works it works but a lot of games have tried and done poorly and it's put the scare in a lot of developers when it comes to taking a chance on something so haphazardly successful.