GuyDuke on 04 May 2015
It is because of the gravity. The Earth's core consist primarily of an iron–nickel alloy, which has a big attraction field that keeps altogether the water and land part of the Earth. It is the same answer as why all planets are spheric bodies.
Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."
