Over extended periods of time and sufficient force all planetary materials, even solid-seeming rock will act like a liquid. Once a body gets to a certain size it is always spherical due to gravity.
Viewed from ~250,000 miles the moon behaves exactly the same as if it were made out of water. This is the reason for its spherical shape.
The Earth is even larger and is also spherical. The surface of the ocean is not flat, it is spherical. If this were not true, one could see all the way to England by climbing up onto the roof of a house on the New Jersey shore. Clearly not the case.
CurlyDave,
Apparently, you are not aware of the vanishing point in visual perspective.
At some point of distance, the vanishing point is reached.
Things become too small for you to be able to see them.
Look it up. “Vanishing point”