Because we're giving you a chance to convince us using some actual sound logic. It's your own fault if you illustrate that you can't even think for yourself by basically just pointing at the Bible. "Why do we need a secular argument?" Because if you're going to convince someone who doesn't share your religion, you need one to even attempt to convince them that you aren't just trying to force your religion on everyone else. And if you're going to try to make something into law, you damn well better have secular arguments for it.
And the fact that people are still arguing about this shows that my version of that discussion is pretty accurate.