Lmao. Seriously though it's exposure; beef, chicken and pork are the cheapest and most common animal proteins available to pet food manufacturers so they're in just about everything. If for some reason the animal has an immune response and those proteins are present at the time the immune system can decide that they're the cause and the animal then develops an allergy or sensitivity to them. With limited ingredient or single protein foods you're just dodging the proteins most pets are pre-exposed to and feeding them one that doesn't cause their immune system to think they're under attack.

One of our cats is so allergic to poultry (chicken, eggs, etc) that he'll scratch and chew himself bloody if he eats it consistently. We had to switch to rabbit and fish based foods or he was an unhappy creature. Just giving him a cat treat with chicken as an ingredient has him scratching his neck immediately and he'll have a couple of fresh scabs by the next morning.