Why isn't Myythic a "Vidoe Game"?
The myythic.com site is a tool that is used in a table-top setting RPG.
A video game, even role-playing video games, have preset obstacles to over-come. Some of these obstacles could be villains that need to be killed. Video games keep score and stats of your objectives as well.
The myythic.com site is one of the unique aspects that helps to make the game run without the game needing a GM.
Think of it this way, the myythic.com site is simply an online adventure that you role-play to in a classic table-top style.