Because they are all fur babies. Always.

My cat was an "old man cat" for a while before he passed.

Anyhow, a "man cat" just sounds like a cat who dresses up like a man and fights crime

I’ve read a lot about how dogs have evolved to hit some of the same triggers that human infants do. I’ve also read about how we tend to view pets as children in our minds. I imagine that that all plays a role in it.

IMO:

Boy/Girl cat implies gender.

Man/Woman cat implies anthropomorphism.

You can call your cat anything you want, they still won’t come.

because they stay smol!

I can hold onto many feelings and ideas at once.

My orange baby boy is an old man.

Because kitties are forever babies 🤗

Generally speaking, the terms man and woman are reserved specifically for humans. I couldn't tell you why, but I suppose it doesn't really matter.

For pets, the use of boy/girl probably does have a lot to do with how people tend to infantilize their companion animals.

Additionally, the boy / girl terminology is often generalized to cover all animals, particularly when adults are interacting with children and by extension when children are interacting with each other. It's not uncommon to have a child ask something like "is that a boy rabbit or a girl rabbit?" but it is a little unusual to hear an adult ask another adult that same question, unless it's sort of tongue-in-cheek or maybe in the presence of kids.

He's just a lil guy

The only true man cat is Idris Elba in Cats

Same reason I refer to my 14-year-old dachshund as a puppy.

Because he's my puppy.

Human/animal bonding is aided because their faces retain neotenic features that remind us of babies. So many are inclined to relate to them like children.

From that article:

Accumulating behavioral and neurophysiological studies support the idea of infantile (cute) faces as highly biologically relevant stimuli rapidly and unconsciously capturing attention and eliciting positive/affectionate behaviors, including willingness to care. It has been hypothesized that the presence of infantile physical and behavioral features in companion (or pet) animals (i.e., dogs and cats) might form the basis of our attraction to these species.

It has been hypothesized that both behavioral and physical infantile features present in companion animals might form the basis of our attraction to these animals and may bear some part of the responsibility for our motivational drive to pet-keeping and pet-caretaking (Archer, 1997).

Not a lot of cats grow to be 18 years old.

Cat smol.

Boy and girl cats grow up??

They do? Who does this? I'd say ask them.

I use the terms "male" and "female".

My cats are gentlemen and ladies

One of my cats is a man cat. We always call him a man unless it Big Bubba Boi for the alliteration. The other cat is a demon.

Because they get neutered/spayed

Define "grow up". Their personality changes with age but they remain fixated on their care giver(s) similar to children for as long as you let them.

my cat always acts like she's a year old and is pretty small for a cat (we assume she's the runt)
when we got her the pound said 2 years old (info from previous owner) but when we took her to the vet we got told that she's actually about 5

She's about 7-8 now and still looks just as adorable as when I got her!!

edit: this image also shows her bent whisker that got messed up when she tried to stick her head in the food container as I was shutting it a couple weeks ago lol

I have a little man cat, he’s my little guy.

My 16yo cat is definitely a prim and proper old lady. Oddly, I call the 4yo Maine Coon "little man" even though he's easily 4x the old lady's size

I'm probably forever going to call my dog "bestest girl", but she's starting to get a grey muzzle too

Because that would be a cattasrophe!

When you're 50 years old and start dating someone do you call them your wo/manfriend?

I have a dead cat. 🤷‍♂️

Probably the same reason my six year old dog is still my puppy. She is just a baby after all.

I think for most people the answer is that pets are not considered peers. They have gender but we will never consider them on a level with human adults - maybe on a level with human babies or young children. We love and prize them, but we don’t give them equal rights and respect. Just like with children.

It probably doesn’t help that we also spay/neuter them at birth, which not only prevents them from ever becoming reproductively viable adults, but also affects their hormonal development permanently. I’m no biologist but it may be accurate to say they never fully mature, even if they do eventually age.

FWIW I personally have two human kids and I refer to our German shepherd as the 3rd grownup in the house. I do say “good boy” but I also say “hey man” and call him “old man.”