Science explains things. It doesn't necessarily mean we fully understand things. Science can be very good at predicting but it is not always very good at explaining "why". Gravity is one of those things. It has been tested over and over again and the results are startlingly predictable (using Newton's Laws). So until it is shown that gravity DOESN'T follow that rule, it is accepted.
EDIT : I still haven't heard how a disc can reel....
Additionally with gravity, it is natural with large masses to assume a spherical shape. You look at the celestial bodies with any kind of density to them through a telescope and they are all spheres. It is the shape with the least amount of volume and smallest surface area for a given mass. Something the size of the earth due to its gravitational pull could not maintain a disc shape unless it was spinning very rapidly, but then everything would fly off of it.