Not everyone lives near a Costco. Not everyone is a fan of the environmental cost of their cheap chickens, or whatever. When I lived in San Francisco there was a Costco but it was more inconvenient to get to via Muni than most of the alternatives. Their parking garage is an absolute zoo.
If I have spare time on a weekend it can be picked up far cheaper in bulk
from a food services supply store.
Not everyone has the luxury of being able to store perishable items in bulk. Personally I struggle a bit to store a whole chicken in my fridge. Six and a half pounds (what you'd have to buy to get Costco's $3/lb price) is quite a lot. And if you want to cook that chicken first and then freeze it, you run a high risk of it just tasting weird.
I just checked around and for boneless, skinless chicken breasts:
Sprouts $7/lb.
Safeway $7/lb (more if you don't want the chlorine treated stuff).
Trader Joe's $7.50/lb (but they've gained a reputation for nasty, woody chicken).
Whole Foods out of stock.
Lucky's $8.50/lb.
Mollie Stone's $8.89/lb.
Berkeley Bowl $9.59/lb.
US Foods Chef Store $3.75/lb for *twelve pounds*.
At least out here there's a lot less variability than you're claiming, unless you're buying enough to fill your entire fridge/freezer.
I just checked around and for boneless, skinless chicken breasts:
At least out here there's a lot less variability than you're claiming, unless you're buying enough to fill your entire fridge/freezer.