Because most operating systems *[hash][1]* 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*.


  [1]: https://www.wired.com/2016/06/hacker-lexicon-password-hashing/