Because games will actually have trophies in them from the moment you drop it in your console. But anyway. Nice of Eidos to show support for their product and patch trophies into it. Later is better than never.
Writing on the community message board, a Crystal Dynamics community rep commented:
"The guys at Crystal have been looking into this for a long time because they really wanted to include it. The good news is that I can confirm that a patch for the PS3 version of Tomb Raider: Underworld is being worked on that will enable Trophy Support."</blockquote>
Bonus for those of you who already have the game or intend to pick it up.