I have solved the mystery.

The reason why only 30% of human DNA performs any useful function is that the rest of it is comments.

Once we decode a typical human genome, we see that the contents begin as follows:

   /* HUMAN_DNA.H
   *
   * Human Genome
   * Version 2.1
   *
   * (C) The Omnipotent Creator
   */
   
   /* Revision history:
   *
   * 0000-00-01 00:00  1.0  Adam.
   * 0000-00-02 10:00  1.1  Eve.
   * 0000-00-03 02:11  1.2  Added penis code to male version. A bit messy --
   *                        will require a rewrite later on to make it neater.
   * 0017-03-12 03:14  1.3  Added extra sex drive to male.h; took code from
   *                        elephant-dna.c
   * 0145-10-03 16:33  1.4  Removed tail.
   * 1115-00-31 17:20  1.5  Shortened forearms, expanded brain case.
   * 2091-08-20 13:56  1.6  Opposable thumbs added to hand() routine.
   * 2501-04-09 14:04  1.7  Minor cosmetic improvements -- skin colour made
   *                        darker to match my own image.
   * 2909-07-12 02:21  1.8  Dentition inadequate; added extra 'wisdom' teeth.
   *                        Must remember to make mouth bigger to compensate.
   * 4501-12-31 14:18  1.9  Increase average height.
   * 5533-02-12 17:09  2.0  Added gay option, triggered by high population
   *                        density, to try and slow the overpopulation problem
   * 6004-11-04 16:11  2.1  Made forefinger narrower to fit hole in centre of
   *                        CD.
   */
   
   /* Standard definitions
   */
   
   #define SEX male
   #define HEIGHT 1.84
   #define MASS 68
   #define RACE caucasian
   
   /* Include inherited traits from parent DNA files.
   *
   * Files must be pre-processed with MENDEL program to provide proper
   * inheritance features.
   */
   
   #include "mother.h"
   #include "father.h"
   
   #ifndef FATHER
   #warn("Father unknown -- guessing\n")
   #include "bastard.h"
   #endif
   
   /* Set up sex-specific functions and variables
   */
   #include 
   
   /* Kludged code -- I'll re-design this lot and re-write it as a proper
   * library sometime soon.
   */
   struct genitals
   {
   #ifdef MALE
   Penis *jt;
   #endif
   /* G_spot *g;   Removed for debugging purposes */
   #ifdef FEMALE
   Vagina *p;
   #endif
   }
   
   /* Initialization bootstrap routine -- called before DNA duplication.
   * Allocates buffers and sets up protein file pointers
   */
   DNA *zygote_initialize(Sperm *, Ovum *);
   
   /* MAIN INITIALIZATION CODE
   *
   * Returns structures containing pre-processed phenotypes for the organism
   * to display at birth.
   *
   * Will be improved later to make output less ugly.
   */
   Characteristic *lookup_phenotype(Identifier *i);