Maybe because all this time nobody made an analysis of the Bitcoin transaction graph which prompted an investigation into ways to determine common ownership of addresses . So you can thank Adi and Dorit for that.

Also: http://dilbert.com/strips/comic/2001-10-25/. Many of us have looked at transactions in Blockexplorer, assuming change should be in a random location, but without paying special attention to this, there's hardly any smoking gun saying "This isn't random".