Because only Imu has the power to erase islands and:
- Imu rarely takes action
- Hence no one knows of their existence
- If islands were frequently erased from history, people would know that the World Government has that power, but that's not currently believed to be the case
- The Gorosei cannot ask Imu for help
- They worship and praise her, they don't tell her what to do
- Imu does not work for the World Government
- They rule over the world from the shadows
Imu is like the Soul King in Bleach, only a secret one.
Besides, it's likely that Rocks was actually killed by Imu. It's said that Rocks wanted to be the King of the World. Imu may have erased him only because he dared for her throne.