Each Droyne world reacts to Chirpers differently. Some ignore, some study, some protect, and some, on the fringes, may exterminate. Droyne are not cookie-cutter about this.
As many Droyne worlds are fairly sedate and isolationist, they aren't necessarily going to know that there are Chirpers nearby. Chirpers are also better at hiding than most Droyne precisely because they have no civilization: no structures, no power sources, and the same racial invisibility the Droyne have. Chirper populations could go undiscovered even on a populous world as long as it lacks electronic surveillance in the places they go.
Also, I suspect that a combination of the Droyne worlds not knowing their own racial history (the Secret of the Ancients is not general knowledge to them), and those that do know that secret also knowing more about casting than we, the game playing observers, do, may lead to them either not thinking about it, or being sufficiently unsure about the results that they don't want to risk it.