Think of it like you own a dog.
If your dog kills a kid, you get in trouble.
If your dog kills the neighbors cat, you get in trouble.
If your dog kills a random bear who picked a fight with it, suddenly your dog is a baddass.
Note, the dog is the badass, not you.
Unfair? Eh, maybe.
But it's designed to take away the easily abusable parts of having pet control.
You get kills, because if you didn't, people would abuse pets to pk people without recompense.
You don't get credit for monster kills, because a sufficiently skilled player would kill any monster in the game without trying with pets, due to some inherent flaws in the system.
Am i working on getting those mitigated? Yes
It takes time and there are a TON of systems that are abused by pets, so until then, this is how it will work.
It was either that or start adding pet immune tags to monsters.