Rune of Solitude (in General)

Why is the Rune of Solitude's effect of Enchant Defenses not an addition to it's base value like that Tattoo of augmentation does with Strength and Dexterity?

Because you need to train ED's before the Rune gives any effect. It does not give free ED's. For the use of the ToA you don't need to train STR or DX because you already have 20 str and dx

I know, but when you train an Enchant defense, the Rune of Solitude doesn't add its value to the base value of the Enchant, meaning that Enchant boosting items such as the Cornutham will not work with the bonus given by the Rune.

From the wiki page on RoS:
"Wear the RoS in combination with a Cornuthaum or an Amulet of Focus to amplify the ED bonus even more."

An AoF only amplifies the bonus on the minion wearing the AoF.