The ancient deity known as Shiva exists on the astral plane and intends to cross over into the mortal world to subjugated it to its rule. However, gods are far too powerful to exist on the mortal plane, as their power would disrupt this aspect of reality and ultimately destroy it. As such, gods need to limit their power by taking a human form through a process known as reincarnation. Through this process, a god is born on the mortal plane as a human, but contains the soul and abilities of the deity. The human form is used as a vessel, which is built specifically to hold a god's spirit in order to keep it contained. These vessels are called avatars, and allow the deity to traverse the mortal world without risking its destruction.
No single avatar is able to possess the entirety of Shiva's soul, as the deity is too vast and powerful to be contained in one shell. Therefore, Shiva must reincarnate pieces of itself through several forms, ending with a total of seven. Once all avatars enter the mortal plane, Shiva's soul can be said to have completely crossed over. At a designated time, these seven will combine and reconstruct Shiva's soul, allowing it to take its true form in reality. This being would be known as the super-messiah, the full manifestation of a god made flesh. With the deity at full strength, it can now rule the plane and exert its true power. However, instead of being reincarnated all at once in one specific location, these avatars have been born millennia apart, with many generations passing between them and at various places. This has greatly complicated Shiva's plan, as it has forced the current avatars to wait for their counterparts to arrive, dragging out shiva's goals over thousands of years.
What would prevent a deity from reincarnating itself all at once?