Why is Venus never visible in the midnight sky, unlike planets such as Jupiter or Mars?

Because it orbits inside Earth’s orbit, keeping it close to the Sun in our sky.

The restriction on when Venus can be seen is entirely dictated by its orbital geometry. Venus orbits the Sun closer than Earth does, meaning its orbit lies entirely within Earth's path around the Sun. Consequently, from Earth's vantage point, Venus can never appear far away from the Sun's general position in the sky. If an object is far away from the Sun's location in the sky, it is visible late at night or near midnight. Since Venus is always relatively near the Sun in our sky, it is only observable for short periods immediately preceding sunrise (Morning Star) or immediately following sunset (Evening Star).