Why the rare mob Rendrak doesn’t spawn by himself

Where is Rendrak?
Well, the easy answer is tha we have to lure him to come. It´s not a normal rare that spawns every 30 min.

The area is in Zoramarsh
There are big bats flying around this area called Direwing Predators.
These Predators drops an item called Smelly Mush Gland evey time (100% drop chance)
You need 10 of them to be able to lure Rendrak.