Because it is there. The people from the Ancient and Medieval periods of history would have been astonished at such an answer. Petrarch (1304 – 1374) may not have been the first naturalist in history but his poetry added an appreciation of nature and a love of scenic beauty to human culture more significantly than ever before. In a way, he was to appreciation of nature as Columbus (a century later) was to America. Neither man was the first but both men made their discoveries matter. Jacob Burkhardt, that great progenitor of cultural history wrote of the journey that became the subject of the poet’s major work:
The ascent of a mountain for its own sake was unheard of, and there could be no thought of the companionship of friends or acquaintances. Petrarch took with him only his younger brother and two country people from the last place where he halted. At the foot of the mountain an old herdsman besought him to turn back, saying that he himself had attempted to climb it fifty years before, and had brought home nothing but repentance, broken bones, and torn clothes, and that neither before nor after had anyone ventured to do the same. Nevertheless, they struggled forward and upward, till the clouds lay beneath their feet, and at last they reached the top. A description of the view from the summit would be looked for in vain, not because the poet was insensible to it, but, on the contrary, because the impression was too overwhelming. His whole past life, with all its follies, rose before his mind; he remembered that ten years ago that day he had quitted Bologna a young man, and turned a longing gaze toward his native country; he opened a book which then was his constant companion, the Confessions of St. Augustine, and his eye fell on the passage in the tenth chapter, “and men go forth, and admire lofty mountains and broad seas and roaring torrents and the ocean and the course of the stars, and forget their own selves while doing so.” His brother, to whom he read these words, could not understand why he closed the book and said no more.”
More of Burkhardt’s piece on Petrarch here.