There are 3 reasons though not equal proportional. Spinning is one has this force makes fluffy by throwing all on earth outward. We get a lot of visitors that come and stay. Over meliniums this is substantial so again Spinning insure a evened blanket. Gravity pull and holds things together by a pulling in or toward core. The core is a bit responsible by being flexible from being
Molten. But the most responsible thing for all the round spheres would be vacuum. Vacuum pulls out in a anticollapsing or else all would give way and fall by gravity laws pulling everything toward center making a smaller by condensing planet/star/satellite. There are bodies along with hosting sun(s)/stars also pulling outward and those are factors of persuasion that may or not be considered.