From an outsider's standpoint, mostly because there's no other way to talk to Christians.
The Bible has a long, heavy history as a prooftext and Christians seem to disparage debate if it doesn't relate directly to verses. There was a period in which well-respected Christian theologians referred at least as often to natural theology as they did to the scriptures, but these days, for a variety of reasons, that's unpopular. It's either a discussion of the pertinent verses or its not discussion at all.
This is a valid stance for people of the faith, but it's inconvenient for people who don't believe as you do. Still, however, they may try to speak your language. Nine times out of ten, though, if a person insists on reinterpreting the Bible, they're not interested in reinterpreting the Bible because they don't believe in its authority - they want to convince you of their underlying argument. They're just speaking Christianese for the sake of argument.
As an example, people will argue the verses in the Bible concerning homosexuality not because they have an academic interest in those verses, but because they've got a personal interest but know that every discussion will come back to those prooftexts and those prooftexts alone. Might as well get on with it.