There’s no provable answer at this point. We simply don’t have enough understanding to be certain of the hows.

And, with evolution it is how not why. Evolution is the why. Evolution is the process that occurs as an organism adapts to its environment over generations. If you ask “why babies be cute?”, the answer is that at some point, babies being cute helped a species of an ancestral creature survive better long term, in the environment that it was in. That’s the why.

So, what we’re left with is essentially guessing what pressures led to a trait. We have to guess from incomplete information because most of the things we’re asking about go back hundreds of thousands of years, or millions of years. We don’t have fossils of everything, we don’t have perfect environmental records of every possible pressure shaping the species of the past.

But, we can kinda make good guesses because caring for our young isn’t just a human trait. Most mammals, many birds, some reptiles, etc, have some degree of attention given to their offspring. You can look at the species that invest more in parental care and see that it’s going to be one good way to ensure that the species continues, right? It isn’t the only successful way of that happening, but it definitely works.

From there, we can also see several factors in non human animals that engage in high attention to offspring. One of them is that most of those kinds of animals are limited in how far along in physical development they are at birth (or the equivalent).

Since any given branch of the evolutionary tree is going to come from common ancestors, a lot of our young are going to have similar degrees of traits developed at birth because we’re related. It’s like your cousin having the same color hair you do, just at a more basic scale.

So, you end up with “cute” being a word we associate with a set of traits. Those traits are common to not only our young, but those of other species.

Assuming all of that chain of assumptions and reasoning is accurate, then we find other animals cute because waaaaaay back in the day, we were the same species, and having our brains feel a protective and emotional response to our young was a survival trait in itself.

What we find cute can be measured to a limited degree. A different ratio of facial features, higher pitched vocalizations, bigger heads in comparison to adult ratios. Those features will cause a response in anyone susceptible to that response. And it’s the same response we have to our own offspring, even down to the chemicals released by our bodies when we are in contact with our offspring.

We have the same rush of oxytocin when petting our kitties and puppies and chicks and whatever else you want to say is cute, as we do when holding our own infants and toddlers. It isn’t only oxytocin, but that’s the one you can find articles about the easiest iirc. At least in relation to our pets and baby animals.

Since a lot of the animals we keep as pets retain features more akin to their early lives compared to their wild kin, the going assumption is that our closest companion species evolved along with us, and it was an advantage for them to be close to us, loved and protected by us. So, they probably kept a more babyish look in response to us doing so more often to the ones that had the optimal blend of features that looked babyish, but didn’t interfere with prayer survival activities like hunting or mating.

Wild animals, or rather animals that didn’t co-evolve with humans, didn’t have that pressure, so they don’t retain those babyish features as long, as they only need to be “cute” long enough to be able to survive without their parents.

This means that it isn’t so much that we find animals that grow into predators cute, it’s that the animals we find cute as adults became that way because we loved them into it.