hatcheries make money based off number of chicks sold. 
 
it's hard to hatch chicks, when you cannot get eggs laid.
 
Most 'true' birds of close to the SOP, are average to poor eggs layers-- yet many of the 'hatchery' sourced DP type birds, lay an egg a day, or 6 a week.
 
If you think that the 'hatchery' style chicken is what it is supposed to be, go buy a copy of the SOP- you'll often find qualities in those birds that is not accepted-- most noteably weight.  and as we all know, weight- is pounds, pounds is muscle.
 
Personal experience:
My parent's buy a bunch of "DP" brown egg layers at Farm King each spring, breeds are common- barred rock, orpington, RIR, white rock, N. Hampshires Reds, EE, etc.  We put the axe to 10-15 of these "DP" cockerels in December (they were April hatched).  On the same day, we put the axe to a group of my 'breeder' type Cornish- and it was no comparision-- which one would expect.
 
Now, the real story comes to my half cornish cockerels that were also butchered that day (June- Aug hatched).  Their mothers were either Show quality Ameraucanas, or hatchery silkies.  Now, any common sense person knows that a true Ameraucana female isn't much bigger than  Leghorn, somewhat meatier-- but the only thing that even rivels a hatchery quality silkie for a lack of carcass-- might be a production white leghorn-- and even then, I think I'll put my money to the leghorn for poundage.
 
Long story short, my half cornish, half silkie EMBARRASSED the Farm King DP type carcasses. Granted they are half Cornish-- but in any regard, they are also half Silkie- and on average were 2-3 months younger!