Why does MariaDB still use the "mysql" name everywhere within the file system?
MariaDB is a forked version on MySQL that attempts to be a drop-in replacement.  A good drop-in replacement should work without requiring a lot of configuration and data file movement.  By using the same directories and filenames, it is far simpler to install as a replacement for MySQL.

I haven't tried MariaDB, but I would expect to be able to install it test it, and decide if I want to revert back to MySQL.  While I would backup my databases, I don't expect to have to restore my data whichever option I pick.