Because most operating systems hash passwords once entered (and store them in a database). This means that the entire password is converted into a collection of cryptographic hashes, random-looking strings of characters into which the passwords have been mathematically transformed to prevent them from being misused.
So, once a password is hashed for a well defined, hash there is no way to attack each character. That is circular logic. To attack each character you must know each character. To know each character you must attack each character.