Because I don't want mouse flavored cat food.

And as far as food for the cat, I only buy him the cheapest kibble, as backup so he doesn't eat the chickens. The chickens eat most of the kibble anyhow. Chickens are omnivores and they lay bigger eggs if they eat more protein.

When the cat gets hungry he lets me know by getting aggressive with the hens and I pay him with a raw egg, which he prefers over kibble. He has never injured a hen even though he gets aggressive.

The yard got overrun with rats and mice that got wise to the traps, and since the cat is there, hardly a sign of rodents, although no evidence he has eaten any either.

The cat also corrals the hens up at night into their coop, like a sheep dog would.

It seems we have an agreement worked out that way.