The force of gravity comes from the mass of an object (technically the mass of both objects). Whether the core was solid or liquid would not affect gravitational attraction.
One might argue the earth's mass might change when struck by a large body (moon, proto-planet, massive comet). However if the striking object had enough mass to change the earth's mass then anything living on the earth before that point will not be living afterwards.
As for why are there so many cultures with pyramids, it might be as simple as that is the easiest way to build a tall structure with the materials they had. You are not going to get very far stacking block vertically. Don't get me wrong what they did was amazing and was necessary for architecture and engineering to advance.
Back on topic I believe the Egyptian's were known to keep trees in pots.