Because Venus is a brand new planet. Look at Venus and such was earth before cooling off and water being formed and reaching the surface in great quantities.
According to most of ancient legends, Venus passed by close to earth and located itself in a new orbit, causing great disturbance to earth and Mars.
Even when ancient cosmos maps show planets, those maps are not showing their "distances" in comparison between themselves and in comparison with earth.
Several old legends have been already proved as scientifically sound, and so far, Venus being a new planet in the solar system is the most sounded scientific explanation of the whole.