I think it has more to do with economics than anything else.
Ya got the carcass of a chicken. Make the most of it.
It take a little longer, but that is one of the reasons why I de-bone the chicken. Roast the bones in the oven till good and browned. Then toss it into a crock pot, add water, and other goodies and come back in a few hours and you have a great stock!
A lot of what is sold is stores is bought for the convenience of having to make it yourself.