While eating these packets isn't actually harmful per say, they will pass right through you "typically", it's breathing them that is bad! If you bite them or break them or accidentally inhale them it can cause very bad lung effects.
It's safe to eat but very much not safe to breathe, they say don't eat it just to try and get people to keep it out of their mouths and airways, and avoid chewing which would break the packet.
Silicosis can be fatal, and apparently is a pretty bad way to die since your lungs scar over and can't take in oxygen effectively so you slowly waste away over the course of a few months, unable to breathe.