My 5 year old asked me this question and I was unable to explain why!
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Here’s a fun activity to do with the kid: build a tower as tall as you can before it falls over. Use any materials you like, but I like marshmallows and dry spaghetti. Experiment with different designs; help him notice that the wider the base, the taller the tower can be. Explain also that things fall because gravity is pulling them down.
Now, talk about mountains. Like the tower, if a mountain gets too tall, gravity will pull it down. But gravity doesn’t really pull “down”, it pulls toward the center of the mass (you might need to explain that “mass” is “stuff” (which is actually “matter” and mass is how much stuff there is, but this nuance can be explained in a couple more years)). This is why Australians don’t fall off.
The earth is round because mountains can only get so tall. A cube would have eight mountains that are too tall, and they would get squished down by gravity until we had a ball again. Same with any other shape with corners. Even a flat disk; the edge would just be a really tall and long mountain range that would get pulled towards the center until you had a ball.
Here’s another activity: try to make a square drop of water. Use a piece of wax paper or other hydrophobic surface, and place a drop of water on it. Use a toothpick to try to change the shape of the drop. As soon as you take away the toothpick, the drop will go right back to being round. All the little tiny (so tiny we can’t see them) bits of water (called molecules) are pulling on each other in all directions. This means that any corners get pulled in until you have a ball. Just like all the rocks that make up Earth.
You can also try blowing square bubbles. Same principle.
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