I believe because it is insane.
That's an old apologetic that was given new life from Soren Kierkegaard. One of my favorite arguments for the Incarnation comes from Kierkegaard's first chapter of his "Philosophical Fragments." It is about the unthinkability of the incarnation (or analogously, a poem). It ought to make us wonder about where the idea of the incarnation came from and how and why, etc. Fascinating, even if you're not a believer.
Here is an excerpt:
Now if someone were to say: "This poem of yours is the most wretched piece of plagiarism ever perpetrated, for it is neither more nor less than what every child knows," I suppose I must blush with shame to hear myself called a liar. But why the most wretched? Every poet who steals, steals from some other poet, and in so far we are all equally wretched; indeed, my own theft is perhaps less harmful, since it is more readily discovered. If I were to be so polite as to ascribe the authorship to you who now condemn me, you would perhaps again be angry. Is there then no poet, although there is a poem? This would surely be strange, as strange as flute-playing without a flute-player. Or is this poem perhaps like a proverb, for which no author can be assigned, because it is as if it owed its existence to humanity at large; was this perhaps the reason you called my theft the most wretched, because I did not steal from any individual man but robbed the human race, and arrogantly, although I am only an individual man, aye, even a wretched thief, pretended to be mankind? If this then is the case, and I went about to all men in turn, and all knew the poem, but each one also knew that he was not the author of it, can I then conclude: mankind must be the author? Would not this be a strange conclusion? For if mankind were the author of this poem, this would have to be expressed by considering every individual equally close to the authorship. Does it not seem to you that this is a difficult case in which we have become involved, though the whole matter appeared to be so easily disposed of in the beginning, by your short and angry word about its being the most wretched plagiarism, and my shame in having to hear it? So then perhaps it is no poem, or at any rate not one for which any human being is responsible, nor yet mankind; ah, now I understand you, it was for this reason you called my procedure the most wretched act of plagiarism, because I did not steal from any individual, nor from the race, but from the God or, as it were, stole the God away, and though I am only an individual man, aye, even a wretched thief, blasphemously pretended to be the God. Now I understand you fully, dear friend, and recognize the justice of your resentment. But then my soul is filled with new wonder, even more, with the spirit of worship; for it would surely have been strange had this poem been a human production. It is not impossible that it might occur to man to imagine himself the equal of the God, or to imagine the God the equal of man, but not to imagine that the God would make himself into the likeness of man; for if the God gave no sign, how could it enter into the mind of man that the blessed God should need him? This would be a most stupid thought, or rather, so stupid a thought could never have entered into his mind; though when the God has seen fit to entrust him with it he exclaims in worship: This thought did not arise in my own heart! and finds it a most miraculously beautiful thought. And is it not altogether miraculous, and does not this word come as a happy omen to my lips; for as I have just said, and as you yourself involuntarily exclaim, we stand here before the Miracle. And as we both now stand before this miracle, whose solemn silence cannot be perturbed by human wrangling over mine and thine, whose awe-inspiring speech infinitely subdues all human strife about mine and thine, forgive me, I pray, the strange delusion that I was the author of this poem. It was a delusion, and the poem is so different from every human poem as not to be a poem at all, but the Miracle.
