All the people that I know who use a Wordpress (or other CMS) *don't* have millions of visitors per day....... So why bother with a MySQL DB (that's annoying to create a DB on a shared hosting, etc.) if one can just use ........ a file DB ( SQLite)
Moreover for backup, having SQLite is 100 times easier : no need to PhpMyAdmin and "export" , just backup the server files ...