Because 664 doesn't go that far. 664 turns into 64 once it crosses 264 ... 168 was built as a freeway alternate to one of the business routes. To make things more fun, if you miss your exit onto 64, 168 turns into 464... which actually dead ends onto 264, which you can take back to 664 ...

I don't know what genius decided that all the freeways need to be derivatives of "64."