Why do think a dog or a cat, hasn't straight out mimicked a word by a human. I don't think its do to lack of intelligence (but perhaps a part of it) but it's mostly because they only imitate so many sounds with their "vocal cords."
Almost all animals except humans rely solely on instinct.But to the first one, same sentient level? Real world animals, particularly chimps, dogs and certain others are as smart compared to humans as Pokemon are portrayed to be right? If animals cant say their names, I dont think Pokemon should either Sorry.
If you actually looked at what humans are outside of the other animal kingdom, you realize that humans are the only animals that they have such advanced language skills. And we also have what, cognitive thinking. Sure there are some bright animals out there.
But they lack what humans do.
All Pokémon for the most part, are like humans but a different species.
Like let's say that humans are the most intelligent of the primates, it's why we can talk, it's why our language has grammar, syntax, and nuances that we can understand that no other primate or other animal for that matter can comprehend.
Pokémon are the equivalent of the other top species.
Like Pikachu being a top rodent, that if they tried really hard could speak human language.
However if I listen to a Spanish song a million times, I would only be able to repeat only a few of the words (words that are close to English words). It's a lot like that, that's why it is so hard for anyone to follow other languages they don't understand because a lot of it sounds like gibberish.
Have you ever heard a foreign language and their talking at any pace, and you don't even know where to begin to start repeating what you heard. It might be easy for humans to mimic words they hear, but sometimes it is difficult. Just imagine how hard it would be to repeat a sentence, a paragraph, even a song you've heard millions of times.
So while all Pokémon do have the capability of speaking human, realize that it is a different language for them, and probably even harder for them to learn, than say humans trying to learn another human language.
Plus I think it's easier to understand a language than to speak it.
Anyway ON TOP of all that, I think you're looking at it the wrong way.
It's a popular, and makes sense theory that Pokémon were named after the noises they made.
They aren't saying their name, that's just how they speak, and their names are a result of the sounds they make. But that doesn't work on the Pokémon that do have animalistic noises but then again clearly they are a different case and really doesn't prove the theory wrong. Surely not all Pokémon could've been named after the noises they make.