Cant we add cat contraceptives to the most used cat food brands?

We have to buy cat food for them anyway whereas we dont have the money to get them all fixed. Are there such things as cat contraceptives and has anyone thought to add them to typical cat food you find at the big stores like wal-mart so the overbreeding of cats can be stopped? Then if you really wanted kittens all you have to do is change what you feed them right? Im just curious as to if theres any way besides spaying/neutering them as I dont like that idea.