Here's a random thought:
Why do we feed our pets the same food all the time? Like how do we get away with that? I realize it's made to provide all the nutrients and things they need supposedly. If that is the case, then how is there not some equivalent miracle catch-all food for humans?
We choose to. Main reason, probably because of convenience.
Look at it this way, feeding your pets is no different than feeding yourself. You could theoretically feed yourself like most people feed their pets, but you choose not to. Because you're experiencing the meal you want it to be enjoyable. However, pets don't normally have that choice unless you let them hunt and gather their own food, they're stuck with what's given to them in most cases and because they are attached to their owners and aren't normally certain(in some cases they don't even understand how to) of how to properly get food on their own, they'll accept anything that doesn't taste like utter ***and somehow manages to sustain their existence. You could just as easily feed them like you feed yourself, with different food each day that isn't just some nutritional pellet. Why don't you? Because it's easier to buy the pre-made nutritional pellets that animals will eat then it is to make them a meal just as you make one for yourself.
Lets be honest, when I shop for food I barely know what I want to eat or can properly decide what I should be eating, many people can't imagine trying to shop and feed not only themselves a differing meal each day, but also each of their pets. So they go for convenience and since in most cases we don't know exactly if our pets experience food the same as we do(or we simply don't care) we easily write it off as "Hey at least they're being fed, and they're not complaining!"
You could also argue for convenience money-wise, but at this point I think I've gone far enough with my poor knowledge of the subject as a whole.