If all the pokemon said things like they do in the anime, half of them would still roar or make their animal noises anyways and every time any pokemon that ever "spoke" (Mewtwo, Jirachi, Meowth, Lucario, etc) people would either be disappointed they don't speak or be mad at the wrong VA being used... also they would need different calls for almost every region based on the names not being consistent among all languages
The games way to using sounds is more realistic. Can you imagine running into a deer in the forest and instead of doing what a deer does when it gets spooked it goes "DEER DEER DURR DURR." *runs away* "DURR DEET DEER DEER deer deer deer..."
"Sorry I get confused because the only two options on Gamefaqs are being an entitled whiner or bending over." - Gandalf the Istari
Just to put it into perspective there are 718 (officially confirmed) Pokemon overall. The game also supports 7 different languages resulting in most Pokemon having different names and thus different cries for each aside from Pikachu.
Calculating that, you'd have a total of 5,020 different voice files that would need to be recorded and paid for. Then they would probably need tone variations for each gender.
Subtracting the Male only, Female only, and genderless Pokemon from the equation you have an additional 4,285 cries for a total of 9,305.
That means the company would need to gather at least 9,305 voice actors (assuming no one voices multiple Pokemon) who would all need to be paid and possibly accommodated depending on how they plan on doing recording sessions.
That's not even getting into the fact that each Pokemon would need an intro cry & fainting cry (like Pikachu has) making a grand total of 18,610 different sound files to be added into the game.
A quick google search shows that the pay rate per voice actor can be anywhere from $250 - $4,000 depending on the length of the voice over and size of the audience that hears it. That makes the overall cost $2,326,250 - $37,220,000 just for voices.