In urea, The two "-NH2" aren't amine, they're amide - organic functional groups characterized by a carbonyl group (C=O) linked to a nitrogen atom (N).
Because of the oxygen atom, amides lose their basic characteristics. This is the explanation I got from wikipedia: "Compared to amines, amides are very weak bases. While the conjugate acid of an amine has a pKa of about 9.5, the conjugate acid of an amide has a pKa around -0.5. Therefore amides don't have as clearly noticeable acid-base properties in water. This lack of basicity is explained by the electron-withdrawing nature of the carbonyl group where the lone pair of electrons on the nitrogen is delocalized by resonance, thus forming a partial double bond with the carbonyl carbon and putting a negative charge on the oxygen. "