Because Find my iPhone is only meant to help you find your phone if misplaced, not stolen.

Law enforcement needs static, printable data in order to do their job, because evidence needs to be shown in court and Find my iPhone's information is only real-time, not historical.

Or heck, you can always do what this guy did:

http://preyproject.com/blog/2012/02/lost-found-case-v93l