I keep chickens. A good hen will lay 5 to 7 eggs a week. They will do this even without a rooster being around. If you are running a rooster with your hens, the vast majority of the eggs laid will be fertilised. Roosters are very enthusiastic about their role in life, and will make sure that just about every hen within reach gets her fair share of attention each day. Even so, most laying hens very rarely go broody. That is to say, every day the hen goes into the nest box, lays an egg, and then comes out again. She has no inclination whatever to sit on the egg. That means there is a fertile egg just sitting there. I can go and collect it and put it in an incubator, and it would probably develop and hatch into a chicken 21 days later. Or I can crack it open and make an omelette. If you keep the egg cool, it won't start to develop at all. Somewhere warm, and after a couple of days you would get a blood spot forming maybe, but it would still be fine to eat.

But what you can't do is put the egg in the oven, mash some potatoes and make gravy, and expect to get a roast chicken dinner. A fertilised egg is an egg. It isn't a chicken.