Imagine a timeline stretching back millions of years. Scientists believe that dinosaurs became extinct about 66 million years ago, tens of millions of years before humans evolved from our primate predecessors. Fast forward to the centuries when the books in the Bible were written, a mere two to three thousand years ago. At that time, dinosaurs had been extinct for many millions of years. Continue to fast forward to the beginning of the 1800s. That is when dinosaur fossils were first recognized and began to be studied to the extent that they are now. So the reason dinosaurs are not mentioned in the Bible is simply that the authors and their contemporaries did not have any knowledge of their existence.
However, occasionally fundamentalist groups argue that the Hebrew word tanniyn, which appears a few times in the Bible and is variously translated as venomous snake, sea monster, dragon or serpent, referred to dinosaurs. Mainline Catholic and Protestant scholars disagree, maintaining that they refer to snakes or aquatic animals that were contemporary to the biblical writers.