Because I like to restock with color morphs I don't have. I think it's more fun that way. Also, I never have to worry about a clutch developing when I don't intend it to, and I avoid the drama of picking a good roo for the flock—all males are insta-cull for me.
Switch them out? Since I'm in it for "looking pets" and eating eggs, I just let them go as long as they can. They nest anywhere they want and I just screen them off with garden netting so they don't go chasing the other girls once they get to the territorial phase. My oldest hen is over 5 years...
None of my hens are related—I keep no males, so each hatch starts when I swap a clutch of empties for fertile eggs I buy online. I would bet it's environmental—big space, lots of cover, no males, and I basically leave them be except to clean up and sneak a camera in there once in a while.
So far, yes. Mama makes noises I never hear any other time of the year, can call her chicks to her, and tidbits for them. At over a week old, they're still snuggling under her at night. This is the way it was for the last hatch, too. Hoping it goes this way for the next one!
I've had at least two hens go broody in every hatch I've ever had (I don't always let them try and have to break them out of it). This will be my third natural hatch coming up in a couple weeks. So I don't get why people say they don't go broody.