Several reasons:
1) When I debate/discuss God's existence with unbelievers I always mention that: Doesn't it take JUST as much faith to believe that everything in existence just sprang forth from NOTHING than to have the faith that there is a Creator/higher power that willed things into being?
2) Sort of what Knight was getting at about the moral law being written into everyone's heart (save the insane/amoral). C.S. Lewis used this example: If you're walking along and hear someone screaming for help you will either:
a) run away and do nothing (self-preservation)
b) go and help the individual (herd-instinct)
c) with humans, however, there is that 3rd reaction, that "what do I do? Do I go help and risk death? No! But....that person needs my help and its the right thing to do!" Where exactly does this wrestling of one's spirit originate?
3) Personal experience. I've lived a checkered life. Again C.S. said, does anyone who admits to being an ex-Christian (not just in name) become an unbeliever because they were honestly talked out of it or were able to just but it aside on some intellectual grounds? Don't virtually all ex-believers simply fall away on their own, slowly, without any outside influence? When my mind is on God, when I pray regularly, when I reach out to others, when I do my very best to put my pride on the back-burner, a peace comes over me in virtually everything I do or say. Everytime I've fallen away for a time, anxiety/problems began to intensify. Not to mention the change my mother went through after my father's death; she went from being a shy, weak churchgoer to a strong, faith-filled pillar of strength virtually overnight which she carried forward for the next 45 years until her death last year. (she credits this to an experience she had late one night about two weeks after dad died but I won't share that within the context of this thread)
4) I know this won't be very convincing to the agnostic, but there seems to me just a common sense understanding that something/someone is.....there. It's been my experience, at least, that I find much more arrogance/intellectually superior attitudes among agnostics than I do among those who profess a belief in God. I'm sure both sides here would agree that humility is a virtue whether a believer or not.
My $0.02