It is as much tradition as anything that people raise their own. Everyone had them or knew someone down the road who did, so getting stock was not a big deal. It is only since the 1950's that people could get chicken products, dressed and ready to go, with such convenience as we now see. This, too, has grown to be a tradition.
This ease has led to people not knowing about chickens anymore. Thanks to the natural, "organic food" movement, they have started to "re-discover" them, as you have. Meaning, they have to re-invest in them, both time, money and effort. Much has been lost.
Grown chickens weren't always available, as you may now think. When they were, they were costly, for all the reasons you mention. Simply put, you didn't sell a good chicken, you put it to use. People depended on them for their food, for their life.
When someone DID sell a grown bird, it was most often for two reasons:
1. As known quality breed stock.
The seller of such birds could make good money for them, among folks intent on flock improvement. Many did just that - reputations were made of such stuff.
{{ This is where the hatching egg market started. Once nationwide rail transport was the norm, the egg business was a natural. It is simpler to transport eggs, as opposed to live birds, and they can be sent farther afield with greater ease.
Eventually, shipment of live chicks came along. So you see, it is more the norm to raise them from egg or chick. }}
2. For the stewpot.
People weren't as "sensitive" as they are today. When the hen had played out or was culled for other reasons, it was now a "fowl." Such birds have one last hurrah, as Sunday dinner. There was once a thriving market for these birds and they could be disposed of readily to specialty wholesalers.
I applaud your mathematics. Control of all things with the chicken (indeed, all livestock) is the key to success. This includes knowing your costs and finding ways to work simpler and cheaper.
