> each file named with the username and containing his password (without any file format, just the password, possibly hashed or encrypted)?

Because your advertisers want to know how many users signed up last month, last six months, and last year. When you only consider one use-case for your data, it's easy to consider using NoSQL or the file system to store your data but in doing so you fail to imagine all the other ways you might want that very same data.