Piper Cat II has never really mewed or purred.
Instead she makes chirping and trilling noises when she’s content or being stroked.
Why would that be?
To mess with your mind. It’s what cats do.
By any chance, is she a Maine Coon?
If you had followed The Prime Directive, I wouldn’t have had to ask. 
One theory I’ve heard is that most meowing between cats happens between mothers and kittens, so a kitten who didn’t spend much time with their mother may not meow as an adult. I’ve also heard that some cats just aren’t that chatty, just like some people aren’t. My own experience lines up with the both theories.
I found this article, which contains some tips on encouraging your cat to speak to you. It also contains some more information on why cats “talk” to us, when they might not talk to other cats. I found it interesting.
You have a defective cat.
On a more serious note, chirping and trilling are instinctive noises for cats, but most cats generally only make those sounds when they are hunting. Your typical house cat will often make those noises when they are staring out the window at a bird that they want to catch. The sounds aren’t particularly useful for hunting, leading many people to believe that they are just sounds of excitement, or possibly frustration.
House cats will also sometimes chirp or trill if they want something from you, like food or if they want to go out.
A lot of cats will chirp and trill when they get excited. It seems like your particular cat has extended this to also include contentment. It’s a bit different, but it doesn’t seem like a huge stretch in behaviors to me.
I have two cats, a brother-sister pair. Tina talks like you describe: she sounds more like a tribble than a cat; it’s freaking adorable. Leon talks like a squeak toy. Not like he swallowed a squeak toy; he sounds like a squeak toy. It’s hilarious.
I have Siamese, bro, sis pair. They wouldn’t know how to sound pleasantly cute. Oh, no, full out MEOWing and HOWLing. They even PURR really loud. And they never shut up!
(Oh, shit, they are staring at me. I swear they can read my mind!!)
This is the same cat who stares at me intently from time to time. I worry that she’s wondering what flavour I am. :eek:
Like I said, chirping is a hunting behavior. 
I don’t know if there is a factual answer to this question. Some cats just don’t purr or meow. They vocalize, yes, but at least two that I’ve owned never meowed.
The first, Denver, could make all kinds of other noises though; and when he was trying to get my attention, he would make a sound like a 1970s funk guitar: “Wa!” He never uttered the sustained “miaowww,” like other cats; just a sharp “wa!” He could string them together if he liked: “Wa! Wa! Wa!” on a single breath. But all of his vocalizations were that short and sharp; he could not sustain anything.
Shiloh, who is still with me after almost 16 years, has never meowed. Like Denver, she vocalizes, but unlike him, it is more “meeps and mumbles.” Hard to describe, but again, it is definitely not a clear, sustained, “miaoww.”
Like I said, there may not be a factual answer. We’re dealing with cats here, and as is often said, cats is weird.
Lock your bedroom door tonight. They know what you’re thinking. Plus they hold grudges!
Seriously though, we bottle fed an abandoned baby kitten we found in a parking lot. It never meowed or purred it’s whole life. We thought she might have been deaf. Never could determine it or not.
One of my two lads was totally silent at first. A rescue he was very timid, and just never made any noise. Then suddenly he started to become very vocal, and now makes no end of trills, meows, and purrs like a diesel train. When he is looking for his brother he will often trill.
What is curious is how innate this vocalising might be. Rescue cats have very varying experience and time to learn behaviour.
One curious trick is that when jumping off the furniture, one cat would often make a Ugh noise when he hit the ground. The other was silent, until he became generally vocal, and now he will go Ugh when he jumps off something. But if startled or running outside for some nefarious purpose, they don’t make any noise. Only when just ambling about, and I suspect, only when I’m there.
Naw. Relax. She’s just wondering what part of you would be the tenderest cut.
Does your cat have legs? Because if not, it might be a tribble.
Are you missing a canary in the house?
Our tux trills a lot. She also purrs, but it’s pretty much inaudible.
“Prime Directive”?
I believe he means the obligation to provide kitty pix in any thread that involves Doper cats.
Northern Piper … step away from the cats … this is a matter of life and death … those are cold-blooded killing machines in front of you and you’re on their menu …
Do they curl up with you at night? … they’re listening to your heartbeat waiting for it to stop … so they can be first in line to the buffet …
Do they knead on you? … they’re looking for soft spots to rip open to get at the tender parts inside …
Do they use the scratching post in front of you? … yeah, that’s what they want to do to you …
Have any of your children disappeared recently? … it’s not coincidence your cats had a sudden weight gain …
Run, don’t walk, to your nearest animal shelter and get yourself a half-dozen dogs … your life depends on this …
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I have a couple of cats that when I scoop them up and hold them tight to me … they will coo … most adorable … coo like a pigeon … just the cutest thing …
This book may just save your life! Ella the Hellacat (a calico, what else would you expect?) is very vocal, meows, chirps, purrs, and even barks like a dog!
Charley makes a trilling noise like an angry tribble when she has a dispute with one of the other cats. She used to say “nee-neep!” when she was a kitten but, now that she’s a grown-up cat, barely vocalizes at all.
I used to be worried about her not talking, but her hearing is fine. When she’s in the cat carrier on the way to the vet, she can meow like a normal cat; I guess she just doesn’t feel she has a lot to say to me in everyday circumstances.