Simply because, much of the time, this has nothing to do with science. We're talking about history here. If the history outlined in the Bible directly contradicts the reconstructed history ascertained from the geologic column or genetics, then there's obviously an issue. It's true, I don't think the Bible is where we ought to go to find information about scientific issues. But to call it unreliable when it comes to historical events? That seems a bit out of line.
In fact, this has been a bit of a pet peeve of mine: equating science with history. Certainly, there's the branch of science called "historical science," but let's not confuse that with observational science.
An example: we can reliably predict weather without wondering about storehouses for wind and rain. Ergo, the Bible was speaking figuratively or was not intending to speak about the inner workings of the weather at all (so too with the shape of the cosmos). That's direct, observational science. On the other hand, when it comes to evolution and the age of the earth and universe and so forth, those are less observationally-driven scientific conclusions, and more necessary frameworks to explain data from the past and present cohesively. There is a difference. Hence, natural history.
So there is a real dilemma here. If it can be concluded that the history and timeline presented in the Scriptures contradict the reconstructed history and timeline of science, then there are reasons to revisit the science and see if we can't come up with a better model. On the other hand, there's also good reason to reevaluate our interpretation. So far, from what I've seen, the latter is what usually happens. The science is accepted as golden and the Scriptures are reevaluated thereafter. The Scriptures are rarely used as a guide for piecing together natural history (though the Bible is, after all, largely made up of books of history). So too, presuppositions are rarely evaluated.
Point being, it's more complicated than "Science says this, so the Bible can't say that," or even worse, "Science says this, so the Bible is wrong about that."