If they be dinosaurs, then why doesn't famed historians like Josephus mention such a creature walking the earth at the same time as man?

The point God was making with Job was this: "Since I made both the behemoth and you, Job, and you cannot control even this fellow creature, how dare you think of usurping My place!" ref-Ryrie.
As for the leviathan, the point is to illustrate what a fearsome creature he is. Both creatures would seem unstoppable at that point in history, while a good .458 Win mag would take care of either one of them in our day. Both illustrations are to make a point with Job, and later references to leviathan are used symbolically to further the point.