I think Alan had it almost correct. Between my personal experience and a documentary I watched explaining some cat behaviors, I think it's not a lack of effort, but a tailored response to get us to perform an option. My cat isn't super vocal most of the time. If he wants someone to play with him at 3:30am he can be very loud and heard all over the house. Over the last year or two, he's started meowing very quietly. It'll be high pitched. It may even sound like a squeaky toy or make no sound at all.
According to the documentary, humans anthropomorphize their cats (no shocker there, right?) They say that when the cat mooshes your stomach or kneads dough, whatever you call it, people think they're showing affection. In reality, this is a behavior that stimulated milk to flow when they were kittens. Humans think it's cute, so they pet them when they do it. This encourages it to continue. However, they'll do it without encouragement simply because we don't correct them. It's like a kid sucking on his thumb or drinking from a bottle at 12yrs old.
The documentary also said that when people think cats are bringing them presents of dead birds, snake (maybe not so dead), or leaves, we think they're bringing us gifts. Instead, the documentary claims that they feel like we're poor hunters and they're bringing food back to the other members of their clowder to ensure their survival. There is nothing in my own experience that contradicts this for me. I can totally see my cat thinking I'm the one that needs looking after.
As for vocalization, they said that cats aren't very vocal with each other after kittenhood. Yes, they yowl at each other, especially when fighting or looking to mate, but they said that the vast majority of cat vocalizations we hear are specifically made for humans. The cats meow and get a reaction out of us. Therefore, they continue to meow. The meow can easily evolve. If they meow and we don't respond, they can meow harder or softer.
In the case of my cat, I believe that when his meow was whinier and quieter, my family thought it was funny and paid more attention to him. They still do, by petting him and/or picking him up and saying his meower is broken. They usually do what he wants, such as going to check his food bowl to make sure he can't see the bottom or letting him outside. Either way, his meow has gotten almost silent. However, when he's ignored it gets louder. Try that with your cat. Ignore the quiet meows, not even looking over, and if they get louder, then you can see what the cat wants.