Because by the time man came to be on the earth, circa 4000 BC according to Bible chronology, the dinosaurs as we know them were long, long gone. Millions of years gone, in fact.
The men and women mentioned in the Bible would have known nothing of and have had no interaction with dinosaurs as they did with other animals, therefore, they are not mentioned. Furthermore, the Bible isn’t a zoological work, and while many animals are mentioned in the scriptures, it’s certainly not an exhaustive list.
Hilarious images from creationist publications of dinosaurs in Noah’s Ark are entertaining but have no scriptural or historical basis at all.