The mummy was actually really hard to refine; if not, Jiang Yi would not have taken so long. Furthermore, only he could refine the mummy this way. He had cultivated the Mighty God Technique and had the Power of Heaven and Earth within him.
The Mighty God Race had cultivated a hundred and eight acupoints. As long as all of these acupoints were triggered, the mummy would more or less be refined. However, if it was someone not from the Mighty God Race, even if they managed to refine all the acupoints; the mummy wouldn't move. Without a power supply, how could something dead move?
When the Heaven Domain great family clans refined mummies, they needed to put into place many arrays and restrictions and then use special spirit stones to give the mummy energy. This way, the mummy could move and attack.