Originally, the Dream wasn’t corrupted but because a world tree was placed in Northrend, its roots reached the imprisoned old god. When the tree was discovered, it got cut down but by that point (and unaware of the damage,) it was too late. Thus the emerald nightmare was born in pockets of the dream and bit by bit, as time went on, more of the dream became corrupted.
It was a pillar of creation corrupted with Void magic. Its not the same as Sargeras talking from a distance to the Draenei unable to force them to anything, nor Kil’jaeden to the Orcs for the same reason.
Anyone shot with that corrupted pillar of creation doesn’t get corrupted, they outright die.
The way I see it, being a druid means binding yourself to the dream. If the dream suddenly changes, so can you, it’s an enormous trust exercise and if it goes wrong it goes very wrong.
But it is interesting to think about what it means that the dream itself to be so easily changed on a large scale. It’s almost as if it literally is the dream of a sleeping cosmic entity and can shift on a stray thought.