Because the world is round.
In Tolkien, the world was originally flat. It was only made round in the cataclysm that sinks Numenor. Ever after...well, let's let Tolkien tell it:
And those that sailed furthest set but a girdle about the Earth and returned weary at last to the place of their beginning; and they said: 'All roads are now bent.'I find this fascinating. If we're going to discuss symbolic geometry, then I'd have to say I normally associate circles, spheres, with infiniteness, but here the sphere is the measure of finiteness. The mariners who return "weary at last to the place of their beginning" - what a sense of futility that phrase carries.
Thus in after days, what by the voyages of ships, what by lore and star-craft, the kings of Men knew that the world was indeed made round, and yet the Eldar were permitted still to depart and to come to the Ancient West and to Avallónë, if they would. Therefore the loremasters of Men said that a Straight Road must still be, for those that were permitted to find it. And they taught that, while the new world fell away, the old road and the path of the memory of the West still went on, as it were a mighty bridge invisible that passed through the air of breath and of flight (which were bent now as the world was bent), and traversed Ilmen which flesh unaided cannot endure, until it came to Tol Eressëa, the Lonely Isle, and maybe even beyond, to Valinor... (pp. 281-2)
I can't think of a better thumbnail image of Tolkien's stance toward modernity on the one hand and the mythological past on the other than this Flat-Earthism. We moderns can't help but think of the discovery (or gradually realization) that the earth is round as an advance, but Tolkien is telling us that it's really a loss for humanity, a huge and definitive Closing of the Frontier. The moment we realize the earth is round, we experience a loss.
But it's what he does next that is so intriguing. He proceeds to imagine the period before we realize the earth is round, not as one of ignorance, but as one in which the earth really wasn't round. The earth's flatness in Tolkien is key: it means that those on earth, Elves and Men, are in lands that are contiguous with those of the Angels. All you have to do get there is sail far enough.
Until the cataclysm, and the rounding of the earth. After that, you can't get there by sailing. Not unless you're permitted to find the Straight Road, and that now takes - well, not exactly faith, not in Tolkien. Permission. Which certainly implies a kind of moral Straightness, although sin in Tolkien isn't very well-defined.
Again with that symbolic geometry. The spherical nature of the world does not connote wholeness (see Cat's comment on an earlier Tolkien post here for what wholeness meant to Tolkien and his generation), but brokenness - bentness, to be precise. Straightness is still possible, but it's no longer natural: now it involves transcendence, superhuman heroism or virtue or perception or favor. Something as simple as a straight line is no longer part of the natural inheritance of Men.
A sorry lot indeed.