Most pet food is made by a few manufacturers. They have a lot of labs testing and refining different recipes.
There are foods for fat pets; foods to clean teeth; food for kidney health.
Pet foods have considerable number of patents. Talk to British people about dog excrement, and there are people of a certain age who remember dog poo being white and furry. People want a food that produces a nice firm stool, that's easy to clean up. Anything too loose and they think the pet is ill (even if it isn't).
There's a lot of money in pet food.
Notice, also, that pet food is often a mix of biscuits and meat, or just meat, or just biscuits. These are solid foods. Water is extra. No-one so far has taken the nutrients a pet needs and blended these into a glop.
