Typically, the alterations made to the DHT molecule are to make it more useful in the body.
But the thing I think you are overlooking is that DHT -while a very anabolic and androgenic compound (many times moreso than testosterone)- is subject to deactivation in skeletal tissue (muscle) by the enzyyme 3b-hydroxysteroid dehydrogenase.
Thus in muscle, it largely renders DHT inactive, while in other tissues, it isn't present in large amounts if at all...and this is where DHT exerts many of it's effects (your prostate, skin, scalp, etc...i.e. androgen receptive tissues which lack the 3b-HSD enzyme).
This androgenic action in target tissues also explains the great hardening abilities of DHT derived steroids like Masteron, WInstrol, Anavar, etc...as well as their seemingly low effectiveness in terms of pure muscle and weight gain.
In women, that androgenic action gives us guys a nice fun var-clit to play with (if you're into that kinda girl).
Also, DHT is a strong CNS stimulant, so combine that with it's androgenic activity in target tissue, and inactivation in much of your skeletal muscle by 3b-HSD, and you have the explanation why the DHT family of steroids are always altered from the original DHT molecule- and are usually great for strength, hardness, and aggression - but not for weight gain.