As the title says, now into the 2nd stretch of human intervention in the reproduction of chicken. Here is what the inside looks like now:
It truly is an amazing process.
So why not let the chickens hatch them? Awesome question with a few answers. Some hens don't want to sit on eggs. Yup, we've messed around with hybrids to produce a breed that can lay consistently but has 0 mothering instinct. Some breeders and I'm talking the show quality stuff, discourage it in their flock.
Some breeds do go absolutely nutso and will sit on just about everything which creates other issues.
A couple years back I had a pair of buff brahma banties.
Cutest little things and they would sit. One of them decided she was going to hatch something. So, I let her. What I learned was I should keep her somewhere away from other chickens and away from other eggs as she stole more and would hop onto other nests... she ended up with about 20 eggs under her, set at different times!! So, she hatched one out and it ended up smothered due to no space under her as she was still hatching out the other eggs. So I took a bunch and put them into the incubator. I did not trust her after that. That hatch was the first time I've had to deal with deformities in chicks.
So if I have a hen that goes "broody" I allow it but under controlled circumstances. They also don't lay while sitting, so, no eggs for 21 days and then if she's a good mom she won't lay for another 2 months while she is raising her chicks. Sometimes, a hen disappears and comes back with chicks in tow. When you have many chickens and some are not of a particular look that attracts your eye, you may not notice it's missing. The real bugger about hatching either in an incubator or witha hen is you end up with roosters!!