Neither Venus nor Mercury appear high in the night sky because they are inner planets, meaning they orbit closer to the Sun than Earth does. This close proximity to the Sun has a few key consequences:

* They are always relatively close to the Sun in the sky: From Earth's perspective, Venus and Mercury never stray far from the Sun. They appear either in the morning sky just before sunrise (as "morning stars") or in the evening sky just after sunset (as "evening stars").

* They are never visible at midnight: Because they are so close to the Sun, they are always lost in the Sun's glare during the night.

* Their maximum elongation is limited: The maximum angle between an inner planet and the Sun, as seen from Earth, is called the elongation. For Venus, the maximum elongation is about 47 degrees, while for Mercury it's only about 28 degrees. This means they never appear very high above the horizon.

In short: Venus and Mercury are inner planets, and their close proximity to the Sun limits their visibility in the night sky.