Why are newborn pigs called piglets?

Answer 1

they are little, so they are "piglets" little cats are called " kittens" these type of names are endearing to people, little dogs are puppies, etc.

More

The suffix "-let" is a common diminutive in English. A diminutive is a term that refers to a small version of something large. Another example is the prefix "mini" which has a similar meaning, although when describing animals "mini-" is generally used when referring to size regardless of the animal's age while "-let" refers to an animal's young.

Adding the suffix "-let" to refer to a young animal is a sort of catch-all formation that's used only when there isn't a separate English diminutive. Thus a young pig is a piglet and young owl is an owlet, but a young cow isn't called a "cowlet" nor is a young dog called a "doglet" because English has the separate words calf and puppy.