To add on to other peoples answers regarding the complete nutritional makeup of pet food; many animals can make a variety of the amino acids they need to survive with just a few inputs (like deer and cows eating only (mostly) plants), but some, especially predatory animals, cannot. They get those nutrients from the prey they eat, which in turn got them from the plants.

It essentially comes down to which enzymes any given organism can create, which ones their DNA codes for *or which microorganisms are allowed to exist within their system. Humans can’t make a bunch of these amino acids themselves. Many (maybe all of them, not that far into my class yet) of the reactions taking in place in any living organism are entirely reliant on enzymes to catalyze them; that is, without them these reactions would take millions of years to complete.

BTW there are appr. 37x10¹⁷ (3,700,000,000,000,000,000) reactions happening in your body every second. All of them (or at least a great majority of them) require enzymes to complete.

need a variety of food to survive?

It’s not true.

Boredom feels terrible while it lasts, but it doesn’t kill you. In the end, humans usually start to get creative after boredom.

Oh, and yes, some food industry has found out things and told you things… yes, they were creative :-)

We don’t. Burritos exist.

You as a human could also live with the same food every day if it covered every dietary need. Especially if you depended on someone else to acquire it and had no choice.

There is an evolutionary push for a rich variety of nutrients obtained from a variety of sources, but the mechanism driving that daily “need” for variety is force of habit and desire for novelty. On top of that, some people are happy to eat nothing but junk and have very narrow tastes. How come?

Also, I can assure you, a lot of cats will periodically stop eating a certain brand or flavor and go through cycles. Does it mean the food isn’t really covering their needs or are they just bored of the same flavor every day? Hard to know, but I would argue your assumption about humans being too different from their pets when it comes to variety in their menu.

Asked my GF who’s an aspiring crazy cat lady:

It’s because (proper) cat food is engineered to contain all the nutrients they need. While it looks like a bland mush of only one thing, it’s more like the cat equivalent of having several full nutricious meals run through a blender. The required variety is built in.