Here's why:
* South Park's style: The show is known for its crude humor and exaggerated characters. The voices are a part of this, often intentionally staying childish and over-the-top even for adult characters.
* Voice actors: While the original actors do have changing voices, they haven't dramatically shifted to represent aging characters. For instance, Trey Parker voices Stan, Kyle, and Cartman, and Matt Stone voices Kenny. They have adapted their voices to fit the characters' personalities and age.
* Changing ages: Although the characters have aged over time, they are still depicted as being around 10 years old.
So, while the voice actors may have naturally aged, the characters' voices haven't changed significantly to reflect this.