It's about the risks.
I have cats, the neighbors have cats, and my little chickens would scratch and flap their wings, and drive the cats nuts. Kitties with tails a-twitching, ready to pounce, kept me in constant fear.
My girls were rarely loose in the yard, and very closely supervised, until they were bigger, probably over 14 weeks old. Now the chickens can scare away the cats, but we have a new cat in the neighborhood that I don't quite trust! They range only when I'm nearby.
If hawks or owls are a possible threat, make sure you have places the chickens can run to. My birds have shrubs and lawn furniture to hide under, even a garbage can near a fence can provide them some safety.
I think we have to teach ourselves to be comfortable letting them range.