The reason the Bible doesn't mention life on other worlds is quite simple: When the Bible was written, there was simply no concept of "Other Worlds", i.e. the authors of the Bible were quite ignorant about the fact that there are others star systems and planets.
Of course, they were also ignorant of the exact composition of blood, and a great many other things.
Just goes to show, don't ask the question 'WHY' using the Bible as a starting point. Rather, just ask WHY in general!
The main problem with dinosaurs and other fossil creatures is the very fact that they are fossilized. It's a terrible thorn in the side of the Tower, because geologists can prove quite easily that the Earth is much, much older than the Tower maintains, and the fossils that are there are just more proof. And, thanks to the repeated pattern of simple lifeforms becoming more complex in later strata, it really is very difficult for them to refute evolution in their books. Notice how they always attack the smaller details of evolution, but don't face up to the big picture.