Here's why:
* "Whiter" is the comparative form of the adjective "white." It's used to compare two things. For example, "This shirt is whiter than that one."
* "Much more white" is awkward and not typically used in standard English. It's redundant because "much more" already implies a comparison.
Here are some examples of how to use "whiter":
* "The snow is whiter than usual this year."
* "Her teeth are whiter after using the whitening toothpaste."
* "The walls of the house are much whiter now."