Because there was no grass to eat yet. Grass did not appear until after dinosaurs became extinct.
That's interesting. cwebber is right, most documentaries feature scenes of dinosaurs in forested grassland habitats (i.e. grass plains with occasional trees, something like modern African savannah).
To cwebber: you're right about the translation of "lotus", but I was thinking of lotus plants which are anchored aquatic plants and can't live in too deep water. For an animal to be sheltered by modern lotus plants it has to be completely submerged. It's something I can imagine a hippo doing, but not, say, a Brontosaurus.
The reason I said they don't eat grass is because logically, what's the point of an animal growing so big if it still eats grass? If it survived on just grass it would have to eat an enormous amount of it to support its life processes. And while we do occasionally get fossilised stomach contents, the main bulk of what we know about dinosaurs' diet comes from coproliths, or fossilised dinosaur dung.
The mountains in Job 40 are the same mountains in Genesis 7-8 - yes, the ones the Flood submerged. Were those mountains foothills?
Please substantiate all your tabloid-sounding statements (you know, the dinos and people coexisting ones) with links we can read ourselves, please.
To Jig: Dinosaurs don't fit all the descriptions either. So it's more plausible to assume that they were hippos and crocs, which God playfully embellished

than dinosaurs which Job had never seen anyway. The leviathan in particular doesn't sound like something you see and then live to tell the story, especially if it's a dinosaur or a prehistoric aquatic reptile.