That is something I have never been able to understand, everyone I know who keeps chickens either has cats or their neighbours have cats. Why would the cats ignore the chickens?
Because chickens are strong and have sharp beaks, and even if they manage to overpower a chicken it will hurt. As long as the cat has enough food it probably won't try. A large rooster would be able to win a fight with a cat.
Also, cats usually won't go after anything that's willing to face the cat down, even smaller animals like rats. I have both cats and rats and the cats will watch the rats in fascination, and even sniff noses with the rats when I have the door to the cage open, but the rats show no fear to the cats, and will even rush the cage bars if they decide a cat is too much in "their territory" and the cats respect that. This has held true through 8 different rats so far, and like 6 or 7 different cats (mine and various roommates).
In general, to a cat, "runs away" == "prey" and "comes right up to me" == "equal and/or potential threat"
Partly perception - chickens are taller than cats and with wings spread out they can look quite large.
Partly aggression - chickens are/can be nasty and will attack other animals with sharp beaks. Combine this with the fact that they're flock animals, one cat isn't going up against multiple flying, aggressive, pointy ended foes.