Here's why:

* Central Atom: The central atom in urea is carbon.

* Hybridization: The carbon atom is sp2 hybridized. This means it has three sp2 hybrid orbitals and one unhybridized p orbital.

* Bonding: The carbon atom forms double bonds with oxygen and single bonds with the two nitrogen atoms.

* Molecular Geometry: The sp2 hybridization and the double bond with oxygen lead to a trigonal planar arrangement of the atoms around the carbon. The molecule is flat with the nitrogen atoms, carbon atom, and oxygen atom all lying in the same plane.

In summary: The sp2 hybridization of the central carbon atom and the presence of a double bond create a planar shape for the urea molecule.