I think the biggest reason is because most people live in cities, so the most they can do is get a few hens every year or two from the hatcheries or feedstore.
The next reason is because most people don't quite realize that hatchery/feedstore hens have short production spans, so once the 2-3 year marker hits, and they bottom out on production, the natural response, especially with the hatchery helping by dropping by another catalog, is to order from them again. It's all the point of the hatchery and how they make money . . . Breed some chickens to lay as much as they can, so production shortens to about 3 very good years, then the customers keep coming back for more.
Luckily I got out of that factory line cycle before my first hens even reached that age, and now all my chickens are hatched a new from my own current flock. I occasionally buy hatching eggs for new strains to be added or improvement or even a new breed/color to work on, but otherwise, I don't waste a bunch of money on new chicks every single year or two. If anything, I hatch out some of my own chicks and sell or eat the excess.
If I hatch out someone else's chicks, I'm likely to keep what I want and sell or eat the others, too.
And of course, one thing a lot of people don't realize is, that sure, some "show quality" strains are less productive when it comes to eggs per week, but in the long run, their productive lives are a LOT longer.
Also, I've yet to see my hatchery girls actually beat my Ameraucanas, Araucanas, and Marans with production. All three of those breeds lay just as good.