What do you mean? There is a journal prefix (Nature has 10.1038) and the link usually translates fairly directly to a URL for new articles.
EDIT: Just looked it up in the DOI rules. The registrant chooses the suffix. So the journals are free to maintain 1:1 mapping to URLs if they wish. I assume DOI is popular because people trust that the central entity might do a bit better job with regards to link rot than the journals themselves.