You can buy eggs hatched by CX hens that happened to live long enough to lay eggs. But the breed isn't designed to live much past 8 weeks. Those hens had a birth defect -- a larger heart -- than the breed is designed to have.
Just because those hens laid eggs doesn't mean that the chicken you buy in the supermarket hatched from eggs.
Incubating cloned chicks under sterile laboratory conditions is simply more efficient than messing about with live chickens and their eggs. I have no proof that this is how most CX birds are incubated -- but if I owned a factory and wanted efficient, predictable results every day of the year, this is how I would do it. Once it's set up it's almost completely automated.