The chickens you buy at the store are 6 to 8 WEEKS old. Your cockerels are 7 to 8 MONTHS old. That’s the difference. Veal (baby calf) is much more tender and much less tasty than an old beef. Suckling pig is much more tender and much less flavorful than an older hog. Baby chicks you buy at the grocery are much more tender and much less flavorful than a seven moth old cockerel.
How are you cooking them is an excellent question. The older meat gets (pork, beef, or poultry) the more you need to adjust how you cook it. You cannot fry a seven month old cockerel, it will be tough and dry out. You need to cook them slowly and with moisture. There are some threads about how to cook chicken at the start of the meat bird forum. It might be to your benefit to read those.
If you want to have chicken like you get at the store, you need to raise the broilers that you harvest at 6 to 8 weeks old. That may be what your customers want. It’s what they are probably used to.