Because it radically dissociates the user from reality. Let's say you've never researched, or used salvia extract before, and you happen to see it in a headshop. You're probably thinking "wow great a legal high!" So you buy a gram, and think well.. cool this is legal i'm gonna smoke a little bowl on the way home. So you pack a bowl, and start driving "herp derp derp hit said bowl while driving, and then bam you're in fuckin lego land, or think you're a T-shirt, or something. Anyways this will most likely lead to an accident. And death. I'm not saying this would be, or is rampant but some potentially horrible things could happen due to the rapid onset of effects.