It's a historical unix thing. Things in /usr/games (which were not all games) were frivolous and not essential to the OS, and were distributed as a separate tape or archive so that admins could easily choose whether or not to install them.
I'll also note /usr/games/dm ( https://github.com/vattam/BSDGames/tree/master/dm ) which allowed sysadmins to restrict when programs in /usr/games could be run. Setting up that structure in /usr/bin would be more work to maintain.
