Hi I do not understand why there is no religion stacks exchange but this question is more philosophy anyhow.
Nondualistic eastern religions (Traditionally tantra movement, some Hinduism strands, probably Kabbalah but I only know it superficially, and more recently Yogananda/Eckhart Tolle and others) all posit the view of the universe that there is one god, one consciousness, or light that is perfect and encompasses all.
Matter is consciousness (or god/light) creating itself within itself and enlightenment is when a sentient being remembers (in an experiential way) their artificial separateness and reunites with totality.
This is a very prominent view in Eastern religions and spiritual movements and yet I cannot find a reasoning why consciousness or god or whatever this totality is labeled would want/need/have to create itself within itself. Some authors label is as love or play which does not go into why. Others claim this is how consciousness grows and evolves by giving sentience free will. Osho probably has the most satisfactory answer I've encountered so far in that he is honest that there is no explanation available to us and one of the reasons many masters never talked about it even positing it as so.
Looking into biology, astronomy or psychology I cannot find reasoning why a system (sentient as in a human brain or non-sentient like an ecological system) would self-limit to grow, or any other reason. The human brain expands neural pathways and in general common sense intelligence is increased through unification of previously separate elements. If there were an all encompassing consciousness I would think it would resemble a sci-fi cloud with infinitely fast transmission of waves that would never need to create matter.