All are made of rock and metal. For the Earth, we can directly determine its composition. We can dig holes and see - but deepest hole ever dug is just 12 km. Volcanic eruptions can reveal something about interior composition. But these methods do us little good for studying other planets.
Density is a convenient and easy way of estimating interior compositions.
Half-way between 3 and 8 is 5.5, nearly the density of Venus. You could come up with other possibilities but this simplest guess is believed correct. Venus is composed of about half rock and half metal. It is easy to do these density calculations for all the planets. Jupiter, Saturn, Uranus, and Neptune are all similar.