We don’t

Everyone is saying "we don't," but do need a diet that's more varied than cats? (for example) If so, why?

I'm a living proof that you can eat the same thing every day for decades and be just fine.

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. 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. 37x10^17^ (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.

We don't. Burritos exist.

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 :-)

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.

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.

Cats and similar animals are adapted to specific environmental niches, but humans are generalists. One of the drawbacks of being generalists is that we’re not specialized enough to fully subsist on any single food source.

We don't need a variety of food to survive. But, generally, we have choice, so we choose to vary our diet because it's more interesting.

Pets do not have a choice. They eat what they're given. Or they choose not to and die (a lil cat I was sitting chose that route).

Wildcats (tigers, lions, bobcats, etc) will take down a prey animal. We think they just eat the muscle. In reality, they often go for the stomach of the herbivore they just brought down to get the vegetable matter there. Then they eat other internal organs (liver, spleen, kidneys) so they aren't just eating muscle.

For our pets, well, we all know they don't eat the same thing every day. Firstly, the the thing they do eat every day, pet food, has various nutrients included so it's a balanced meal for them. Secondly, we give them treats which may or may not be beneficial.

As for we humans wanting variety, it's exactly that. We want but don't need as much variety as we get. We enjoy the different flavors even if the items containing those flavors aren't exactly good for us (twinkies, 8 year scotch, etc). Our pets and wild carnivores don't get the opportunity to try these other flavors (well, our pets get some opportunity but not to the extent we have granted ourselves).

If you think about it, for the past hundreds of thousands of years we've been regionally locked in our areas so variety is a loose term.

I have a low burnout rate with food, and there are probably meals that I could just keep eating repeatedly. Note that these are multi-ingredient foods so could theoretically offer balanced nutrition and be flavored to a preferred taste.

I don't do so for several reasons: cost, availability, convenience, sharing meals with others who have different food preferences, and simply because I still prefer variety.

Kongbap says nah, it's all you need to survive.

Not sure that's a universal thing with cats. Unless she's really hungry, the current feline resident of my household most definitely will turn her nose up at a dish if it's the same as she was fed in the last meal.