This is because a planet has unique, internal characteristics or composition that are settled at different depths based on their given density.
Taken, for example, our own planet Earth. The Earth's structure is well determined. It's interior consists of several layers: the crust- a relatively thin region of low-density silicates; the mantle- a thick region of higher-density of iron-rich silicates, and lastly is the core- the central region of iron which is mixed with various impurities.
