Because standing on the Earth and looking up into the cosmos with only the naked eye and nothing else to aid our observation, leads one to believe that the earth is static and the heavens swirl around us, not that the earth rotates and revolves around the sun. For millennia, people actually believed the Earth was flat and the sun traveled underneath the land at night, which begat the concept of an 'underworld'.
It took until the early 16th century, until Copernicus formulated that things worked much differently.