Drupal Database Relationships
Quite a vast topic among many reasons:


MyISAM doesn't support foreigns keys and Drupal used this as a default engine until D7 (which now use InnoDB as default if available).
Schema now support foreign keys so you can benefit from this in documentation.
Enforcing FK in Drupal would be quite a challenge (Damien Tournoud - one of Drupal most prolific core contributor - stated: Drupal is far from being able to use foreign keys properly).