Because past the phenomenon Beast realm, Demon beast gained intellect that made them capable of ignoring their instincts and acts rationally for the purpose of survival, the forests were far from being adequate homes for them anymore.
Now they could reason that humans would always hunt them down as long as they could be overpowered. Staying in the forest was only going to make that easier.
However, without being at the Aurora beast realm, they could never take human form and attempt to mingle and once they advanced to the Aurora beast realm, somehow the Exterminators always knew and would come down to offer two choices.
Serve the Haron empire or die.
As for humans, once they were in the Cosmic Phenomenon realm, only meats of beasts at the corresponding realm could truly have desired effects on their bodies and cultivation. And while a scant few skewer and eat them anyway, the majority felt it was much too close to cannibalism.