Roosters aren't kept with chickens used for egg laying - it's not cost effective to search every egg to see if it's been fertilized or not and most customers get pretty upset if the crack open their morning egg and end up with a partially developed chick fetus in the frying pan. Roosters also aren't necessary to a flock of chickens - they do help defend against (smallish) predators but in a human kept environment that's often not necessary. A farm that breeds its own chickens may have a rooster or two that is introduced to a hen every now and then (and that hen is kept separate from the egg laying hens until her chicks have hatched). And, many farmer do buy chicks commercially - because when you use the 'natural' method, you often end up with too many roosters in the clutch and roosters are useless unless you're going to slaughter and eat them. When you buy chicks, you can specify that you only want hens and the wholesale seller deals with the excess males (generally in a very gruesome way, I don't recommend googling this).